How to put a fixed start date on my calendar?

Hi there. Sorry for asking such a basic question. I’m looking for an expression that helps me put a start date to my application’s calendar. Currently, the data information is obtained from {“FromDaysInThePast”: - 90, “ToDaysInTheFuture”: 90}. However, I would like the start date to be, for example, “07/01/2021”. I can’t find a way to do it.
Also, I would love for you to finish collecting the information on the last day of the third month after today.
If someone could help me with either of the two expressions, I really appreciate it.