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