Is it grammatically correct to say "it is 2 months since we've been dating"? Because I heard with "since" we must use a perfect or/and a past tense, like "it's been 2 months since we've been dating".
Although you may have heard that, I don't know of any reason why it would necessarily be impossible to use a present-tense verb before since. The Oxford English Dictionary lists a number of examples of this supposedly impossible usage:
1807 Wordsworth Poems I. 23 I'm as great as they,..Since the day I found thee out.
b. Used in place of ‘that’.
- a1645 W. Browne tr. M. Le Roy Hist. Polexander (1647) ii. ii. 194 It is..five moneths now, since these honor'd personages have suffer'd..indignities in these Dungeons.
- a1774 O. Goldsmith tr. P. Scarron Comic Romance (1775) I. xxviii. 313 Though it is now fourscore years since he has plagued all those who have any dependence on him, yet he is so well in health [etc.].
- 1804 C. Smith Conversations I. 162 It is near four months since Ella has been away.
More recent examples can be found by searching Google Books:
The Minister of Internal Affairs should also inform us whether the Government still intends to set up a Board to encourage our secondary industries, as it is two months since the Minister told us that the Government did intend to do so, and there is no indication of such a board so far.
(Mr. KYLE, in New Zealand Parliamentary Debates 1931)
"Since" is defined by Merriam Webster as "from a definite past time until now", so "since" itself is a past tense word. Thus, using present tense before it would be mixing tenses, which is almost always grammatically incorrect.
Basically, there might be some case out there where using present tense before "since" would be acceptable, but it would probably be a very odd scenario. In most examples I can think of, the use of present tense would not be accurate.
Also, just keep in mind that, in your scenario, "it's" is a conjunction for "it has" not "it is", where "has" is the past tense form of the present tense "is".