Is this sentence grammatically correct?
I will be available anytime until 12pm or after 8pm from 2nd of April.
I am available anytime until 12pm or after 8pm from 2nd of April.
Both are grammatically correct.
In distinguishing between them, You would "typically" use the first example for a single event, and the second example for a recurring series of events.
For instance, if you want to schedule a meeting with your boss to discuss something specific, you would use the future tense. ("I will be free . . .")
On the other hand, if you are a professor who has posted weekly hours, you would use the present tense. ("My hours are . . .")
However, there is no rule that says you have to use one or the other. It's just a matter of convention and preference.