I recently wanted to see what happens when I change my local time to something obviously wrong. I tried the year 2218, so 200 years in the future. The result: I couldn't access any website anymore (I didn't try too many, though). I got this error:

enter image description here

I guess NET::ERR_CERT_DATE_INVALID means that an HTTPs certificate is not valid. But usually there is an "advanced" option that allows me to ignore it. Not so here. Also, I wonder why it says "your clock is ahead" - if chrome knows the correct time, why doesn't it take this for comparing?

Coming to my question: How important is local time for security? If an attacker can arbitrarily change the system time, which kinds of attacks allows this? Are there reported cases where time manipulation was a crucial part?

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    "if chrome knows the correct time, why doesn't it take this for comparing?" - I don't know how it's checking that, but 200 years in the future is rather obviously incorrect. It's possible to know that something is incorrect without knowing what the correct data actually is. Sep 11, 2018 at 18:41
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    Probably chrome does not know the correct time, but knows that the certificate validity is way out of range.
    – allo
    Sep 12, 2018 at 11:50
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    @Acccumulation As I said, I've not bothered to check how it's implemented. Could be if it sees a certificate that expired more than 10 years ago it just assumes your clock is wrong. Sep 12, 2018 at 15:55
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    @Acccumulation It could be based on the current version's release date as well. Sep 12, 2018 at 17:17
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    @AndrolGenhald I would guess that Chrome itself has a time lookup with its own time servers built in. I'm sure it uses the local time, but I wouldn't be at all surprised if a network time lookup happens in Chrome to know the "real" time regardless of what the local time says. Sep 12, 2018 at 18:58

9 Answers 9


If an attacker can arbitrarily change the system time, which kinds of attacks allows this?

Beyond certificates...

Poorly seeded RNGs

They may be able to exploit a poorly seeded random number generator. Using time as a seed used to happen a lot before better random number interfaces realized your average programmer can't be trusted to provide a good seed. An attacker can exploit this by setting the system time to a time where the random number generator will produce their desired output.

Of course, the attacker can save themselves a lot of hassle by simply waiting for a desirable time.


UUIDv1 and v2 both depend on the MAC address and the time. The MAC address can be discovered. Being able to set the time means they can now control which UUID is assigned next. For example, they may be able to duplicate the UUID of an administrative account for themselves. Of course, UUIDv1 and v2 are not meant to be secure, they're just meant to be unique. If you want secure and unique you use UUIDv4, but there's plenty of software which uses UUIDv1 and v2 inappropriately.

Periodic jobs

Many systems have critical processes which run periodically at certain days and times. The attacker can mess with the time to manipulate this.

Many systems have maintenance windows which occur at certain times. The attacker could continuously reset the time to remain in this window and keep the system down for maintenance.

If there is a resource intensive process which occurs periodically, they can mess with the time such that multiple of those processes are running at the same time. If the system does not limit the number of simultaneous processes this can swamp the system.

Or you can go the other way and continuously reset the time to prevent a critical maintenance process from running.


The system may have watchdog processes which look for too many or too few actions happening in a window of time. The watchdog may choose to take automatic maintenance action such as restarting machines or stopping services. An attacker can manipulate the time to make it seem as if the rate is too high or too low tripping the watchdog and causing it to halt a working system.

  • Source for UUIDv4: Section 4.4 Sep 15, 2018 at 11:22
  • +1 Good answer, but I'm not sure I totally agree. 1) cryptographic RNGs are seeded from things other than the clock, ex.: previous seed, disk IO events, network packet events, so full control of the clock will not give your full control of the seed. 2) even UUIDv4 is not intended to be secure: "4.4. Algorithms for Creating a UUID from Truly Random or Pseudo-Random Numbers" so any software which uses a UUID as a client secret is probably bad software. 3) Great point about periodic jobs and watchdogs, another good example is advancing the clock to skip over the antivirus' scheduled scan. Sep 16, 2018 at 13:33
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    @MikeOunsworth Sure, if you're doing it right the first two aren't a problem, but... 1) The amount of code that is seeding with time will shock you. 2) Lots and lots of software exposes their UUIDs.
    – Schwern
    Sep 16, 2018 at 18:30
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    @Schwern Good discussion! 1) srand doesn't claim to be a cryptographically secure RNG. srand(time()); is fine if you're, for example, generating texture maps; if people are using srand to generate cryptographic secrets then yeah, that would be a problem. 2) I'm not sure why that's a problem? In our software we base our security model on the assumption that attackers can figure out UUID values if they really want to. So if a developer needs to expose a UUID for whatever reason, whatever. UUIDs (like any other kind of ID) should not be used as an auth token. Sep 16, 2018 at 19:27
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    "In our software we base our security model on the assumption that attackers can figure out UUID values if they really want to." That's excellent. A lot of software does not. These are attacks which exploit common, small security mistakes.
    – Schwern
    Sep 16, 2018 at 19:32

You have a bunch of questions rolled in there.

I guess NET::ERR_CERT_DATE_INVALID means that an HTTPs certificate is not valid.


Here is the cert for help.ubuntu.com:

Certificate for help.ubuntu.com

You'll notice that it has Valid From and Valid Until dates; if you try to access a site protected by this cert outside of these dates, your browser will complain. The reason certs expire is (among other reasons) to force webmasters to keep getting new certs using the latest crypto and other new security features in certificates.

When your browser is trying to decide if it trusts a certificate, it uses the system clock as the definitive source of truth for time. Sure, it'll try to use NTP, but if you (the admin user) have explicitly told it that the NTP servers are wrong, well, you're the boss.

If an attacker can arbitrarily change the system time, which kinds of attacks allows this?

Let's consider personal computers and servers separately. I haven't done any research here, just off the top of my head.

Personal Computers

  • Users often play games with their system clock to get around "30-day trial" type things. If you're the company whose software is being used illegally this way, then you would consider it a security issue.
  • Spoofed websites. It's much easier to hack old expired certificates -- maybe it used 10 year old crypto that is easily cracked, or maybe the server was compromised 6 years ago but the CAs don't track revocation info for that long (idea credit: @immibis' answer). If an attacker can change your system clock then you won't see the warnings.


  • Logging. When investigating a security breach, if your servers' clocks are out of sync, it can be very difficult to piece together all the logs to figure out exactly what happened and in what order.
  • Logins. Things like OTP 2 factor authentication is usually time-based. If one server's clocks are behind a different server, then you could watch someone enter an OTP code, then go use it against the server that's behind because that code won't have expired yet.
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    Also, obviously, denial of service. If the server uses some third-party web services by HTTPS, the requests will start to fail just like the browser did above.
    – IMil
    Sep 12, 2018 at 3:25
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    w.r.t. logins, not just OTP 2 factor auth is affected. Kerberos requires computers to be reasonably in sync ("to prevent replay attacks"). Windows domains, using Kerberos, for example, require computers to be within 5 minutes of each other (by default) or you can't get authorized on the domain.
    – davidbak
    Sep 12, 2018 at 5:35
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    Another issue, brought up at this year's DEF CON, is the fact that someone may register a website that was previously registered and may even still have a valid certificate that the previous owner has access to. The expiry date ensures that this will not go on indefinitely.
    – forest
    Sep 12, 2018 at 20:34
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    @bradbury9 Maybe. Browsers will complain about old crypto (ex MD5 / SHA1) or missing security features (Certificate Transparency), if the victim is using an up-to-date browser. I could be wrong, but I think it's actually the browsers, not the CAs, that drove the decision for a 2-year max cert lifetime, and the CA with the shortest cert lifetime - Let's Encrypt at 3 months - is completely free. Sep 14, 2018 at 11:20
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    Upvoted for "well, you're the boss". Sometimes users can be their own worst enemies no matter how good the security system. :-)
    – Ben
    Sep 14, 2018 at 18:39

One reason is that certificate revocation records are not kept after the certificate expires.

Suppose I stole Google's certificate 10 years ago. Google immediately noticed and revoked their certificate. Since the certificate expired some time in the last 10 years, the revocation entry was deleted. If I set your clock back 10 years to when it was valid, I can impersonate Google and your browser won't notice, because it won't know it was revoked.

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    It seems to me you describe the reverse of the question. OP shows when you change the local time, the browser accepts the change and act accordingly. If anything, this behaviour makes the attack you describe possible, it does not seem to prevent it.
    – Suma
    Sep 13, 2018 at 8:39
  • @Suma, for security purposes Chrome will always use the user's computer's time. The browser says "This cert expired 2018-12-09 and it is currently 2218-08-06, so the cert cannot be trusted and a certificate error page is needed". After determining that a certificate error page is needed, Chrome tries to create an appropriate page, by guessing the root cause of the problem. In this case Chrome guessed that maybe the certificate is not actually 200 years old, maybe your clock is wrong. So Chrome suggested that you change your clock.
    – Buge
    Sep 15, 2018 at 23:14

Your question is broad, but if an attacker can change the local system clock, then they can poison the logs of their activity. That way, they can hide their activity to appear to have occurred sometime in the past (and maybe beyond the window that the admins are looking for activity) or to coincide with other user's activity.

For example, if you break into a system in the middle of the night, you can set the clock to be at noon the previous day, do your activity, then set the clock back. Anyone inspecting the logs will assume the normal user did the activity (or not see it at all among the normal user's activity).

This is why setting your clock to be synced with an authoritative external source is important. That, and that all logs from all sources can be properly correlated.

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    This doesn't address why an out of sync clock is an issue in this particular case. Sep 11, 2018 at 23:59
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    It's a broad question and I tackled part of it.
    – schroeder
    Sep 12, 2018 at 7:13
  • Hm... that is only true if there are multiple systems involved, right? Because logs are usually written line by line (appended to a textfile). This means the attacker could "slow down time" without notice, but if the attacker "went back in time" (making the log entries not chronological) it would be super easy to spot the ones where he manipulated time. Sep 14, 2018 at 5:09
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    This is also why some logging solutions include sequence numbers with the entries, or, for example, why linux kernel logs use seconds since boot. Sep 14, 2018 at 16:01
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    @schroeder for i in range(1, len(loglines)): if loglines[i]['time'] < logline[i-1]['time']: print("line {} has fraudulent time".format(i)) Sep 15, 2018 at 11:12

There would be two possible situations where a computer that thinks the year is 2038 tries to connect to the outside world, and everything in the outside world says that it is 2018:

  1. The computer's clock is wrong.

  2. The year is actually 2038, and everything in what seems to be the outside world is being faked, using certificates from 2018 that have been cracked in the 20 years since then.

Although attacks of that form would be hard enough to pull off that the first possibility would in practice be far more likely, guarding against the second would require getting confirmation that the computer's clock actually is wrong.

  • Hm. And where do you draw the line? What is an acceptable difference to the outside world? Sep 14, 2018 at 5:16
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    @MartinThoma: If a computer's clock is within the range where its certificates are valid, then the certificates are probably valid. If the computer's clock is outside that range, it's possible (and in some cases likely) that the certificates are actually valid and it's merely the computer's clock that's in error, but a computer can't be certain that any external time server is trustworthy without a certificate that's valid at the computer's reported time. If external time servers consistently report times that are "in the past", that would suggest that...
    – supercat
    Sep 14, 2018 at 14:35
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    ...a diagnostic about the computer's clock being in the future will be more helpful than a message saying all the certificates have expired, but should allow for the possibility of a user looking at a message "Your computer's clock says it's 2018, but everyone else says it's 2012" and saying "It is 2018; the servers claiming 2012 are wrong--probably deliberately so."
    – supercat
    Sep 14, 2018 at 14:38
  • So your line is a calendar year. This means 365 days wrong in a leap year would be ok, but one day more is not. Sep 14, 2018 at 15:06
  • @MartinThoma: What times would be acceptable would depend upon what certificates the computer happens to have installed at any particular moment.
    – supercat
    Sep 14, 2018 at 15:23

If you are running a single Windows Domain Controller, or part of it's network, a time difference of only five, or a few minutes can be critical: Kerberos Time Tolerance

I once had to deal with a failed Windows SBS 2008 Server and had to find a way to get in as SYSTEM to read the Event Log.

It turned out that a power surge had reset the CMOS BIOS clock on the Domain Controller, back to the system design date, some years before. The result being that Kerberos tickets could not be issued.

Kerberos tickets are valid for just a few minutes.


Another (slightly convoluted) example of an attack against a computer with lagging system time is serving outdated DNS records from a zone secured with DNSSEC.

For example, if client asked for www.example.com address, the attacker could reply with a referral to his own DNS server and an (expired) proof of non-existence of a DS record for example.com from the time when example.com was not yet signed, and subsequently serve forged DNS records for anything in example.com zone.


It could be considered faking and not having the computer clock synced always could have its advantages and disadvantages. Obvious ones are those already pointed. One advantage is websites will not know your real date, although this would enter more into privacy than security per se, normally privacy is considered part of security.

I don't know much about NTP enumeration but its used in pentesting so it's a factor to take into account for security.

"The Network Time Protocol is a protocol for synchronizing time across your network, this is especially important when utilizing Directory Services. There exists a number of time servers throughout the world that can be used to keep systems synced to each other. NTP utilizes UDP port 123. Through NTP enumeration you can gather information such as lists of hosts connected to NTP server, IP addresses, system names, and OSs running on the client system in a network. All this information can be enumerated by querying NTP server." source: "https://www.greycampus.com/opencampus/ethical-hacking/ntp-enumeration"

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    This answer lacks concrete details to be useful to me. What is a scenario where it does harm that websites don't know the local time on my machine? How is that a privacy issue? Sep 11, 2018 at 20:24
  • If you are using a vpn or proxy and your system is using a different time zone than what your ip should have... then any person would consider you are using a vpn or proxy. Most proxies and vpns are not able to hide the time clock of your operative system by default and websites can read that (therefore pointing to the place you live even if using a proxy/vpn, therefore breaking down your privacy) Sep 11, 2018 at 20:47
  • @DevuanUser there are a lot of conditions that need to apply: 1) the site would have to look up your IP against the location and look up the timezone, 2) the javascript on the page would have to query local time and send it back to the server, 3) your VPN would have to be in a different time zone than you are. These are important conditions and should not be glossed over in such a short description.
    – schroeder
    Sep 11, 2018 at 21:01
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    "One advantage is websites will not know your real date" — your real date is the same as everyone else's. We're in the same universe, and we have just the one agreed-upon time stream that we hold in common. Like space. (Exercise: I'm going to conceal how many dimensions I'm in right now, for security purposes. What? You guessed three? Darn it! How did you know?)
    – Wildcard
    Sep 12, 2018 at 0:35
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    NTP protocol only exchange timestamp in UTC, no matter the settings of the client & server. So the most an NTP server might get is the machine from a certain IP is off-sync by certain period. Ironically, if it's consistently out-of-sync by a fixed period, the data can be used to track the machine even after IP address change. Compared to regularly synced machine that will have similar offset period with other machines in the region.
    – Martheen
    Sep 12, 2018 at 4:34

Many software, especially those that are working in a cluster, time sync is very important. So if someone messes with the time then the software running on this system will cause a lot of issues, especially with sync. This will eventually count as service impacting.

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    Welcome! This looks more like a comment, than a full answer. I recommend removing it and adding it as a comment on one of the other answers, or add enough information to make it into a full answer.
    – browly
    Sep 13, 2018 at 22:51
  • I think for writing a comment you must have at minimum 50 points: stackoverflow.com/help/privileges Sep 16, 2018 at 6:04

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