Dates in Moodle are unix timestamps. And unix timestamps use 1970/01/01 00:00:000 as the base date. So the dates in Moodle are the number of seconds that have passed since that 1970 base date.
So the value in mdl_user.lastlogin is the integer representing the number of seconds that have passed since that 1970/01/01 date to the date/time that the user last logged into Moodle.
https://en.wikipedia.org/wiki/Unix_time
So the value in mdl_user.lastlogin is the integer representing the number of seconds that have passed since that 1970/01/01 date to the date/time that the user last logged into Moodle.
https://en.wikipedia.org/wiki/Unix_time