“Monotonic” means non-decreasing (or non-increasing if you’re going the other way). Values are allowed to repeat. The term you’re looking for is “strictly increasing.”
I guess this hinges on whether you think Unix time is an integer or a float. If you think it's just an integer, then yes, you can't get a negative delta.
If, however, you think it's a float, then you can.
If, however, you think it's a float, then you can.