Sounds to me like you are having your first encounter with the Unix Timestamp concept. Basically, all kinds of date-related operations generate and/or accept as parameters an integer value that corresponds to the number of seconds that have passed in the 'linux epoch.' More precisely, the number of seconds (or milliseconds) that have passed since January 1, 1970. You might want to also check out Javascript's Date.Now() function. It returns a similar value.
This returned value does in fact correspond to a specific date. Check out the date function as Weedpacket suggested:
echo date("Y-m-d H:i:s", 1423900800);
The output is:
2015-02-14 00:00:00