Want to enter a date linked to the cell next to it

Say I have two cells, A1 and A2. I want to be able to enter number into A2 and have a date entered into A1 automatically which shows the time and day the information was entered into A2. I was also wondering if there is any utility in using something derived from the UNIX timestamp versus the system clock, or a time from NIST (not sure if that’s entirely relevant but I’m trying to learn).