Have a good day!

Have a great night!


Is this an imperative, as in:
Sit!
Stay!
Go!

I don't see how it is in the simple present, as in the simple present we (and forgive me if I'm mistaken) habits. Such as in these examples:

I play soccer
I grow trees


It also notably not in the present continuous since it doesn't have its structure. It's not in any of the past tenses either....
So what is it?

Thanks!

The simple answer is that it's not a tense; it's a grammatical mood. In this case, as you indicated, it's the imperative mood.

Tenses and moods are completely distinct categories in linguistics.

Thanks Ian. Still, some of the examples in "moods" could theoretically go under tenses, as a neophyte like me sees it.

Is there a rule as to what tenses are versus moods? (In other words, when are they mutually exclusive and when are they mutually inclusive)?

There is no future tense,just combinations of words we use to represent the future; this indicates future but its not a tense.