This is a substitution, not steganography. One of the key goals of steganography is deniability. In information security, deniability is a property which allows denial that information exists or that an action took place. While the meaning of the message is somewhat disguised in your scheme, the presence of the general message is not. Steganography is concerned with building a hidden channel which prevents detection of a message being exchanged covertly within an innocent and unrelated communication.
An early simple example might be the book dot cipher where small dots were placed on letters in a book to turn the book itself in to a message. This primitive approach was not completely hidden, but at a casual glance, it would just look like someone who had a book, it doesn't appear to be any special communication.
To have a linguistic steganography, you would need to encode the information such that a seemingly normal conversation would actually convey some other information. In your example, it is clear that you are talking about a meeting at some time, so the message, while the details are altered, is not hidden.
A better example of an extremely primitive linguistic steganography would be to have the time of day of a phone call encode some information, or perhaps have values assigned to different items to have for dinner. If you called to ask when the chicken would be done cooking, that could mean that you needed to meet up later at a drop point. The actual message is completely concealed by what appears to be a legitimate communication.
Borrowing from Steffen's excellent answer, it seems you may be getting confused by the concept of "synonym substitution" which is a common basic form of linguistic steganography. With synonym substitution, words are replaced with their synonyms to encode information based on if the word changes or not. This could either be through a simple binary encoding or a more complex code word system, but the key thing is that the message being sent is unrelated to the message it appears in. The message that is apparently being sent must be deniable that any hidden meaning exists.
Now, in theory, you could use antonyms instead of synonyms for the substitution used to encode the information, but this runs in to a problem. Synonym substitution works because the paraphrase will make sense and seem like a reasonable paraphrase or mis-remembering of a known quote. If you instead start throwing in antonyms it becomes much easier to tell something odd is going on. Since it looks odd, the deniability is greatly reduced and the message is much more likely to draw the attention of code breakers. This renders antonym substitution to be a relatively poor means of encoding steganographic information as well.