PHP Converting Integer to Date, reverse of strtotime

Yes you can convert it back. You can try:

date("Y-m-d H:i:s", 1388516401);

The logic behind this conversion from date to an integer is explained in strtotime in PHP:

The function expects to be given a string containing an English date format and will try to parse that format into a Unix timestamp (the number of seconds since January 1 1970 00:00:00 UTC), relative to the timestamp given in now, or the current time if now is not supplied.

For example, strtotime("1970-01-01 00:00:00") gives you 0 and strtotime("1970-01-01 00:00:01") gives you 1.

This means that if you are printing strtotime("2014-01-01 00:00:01") which will give you output 1388516401, so the date 2014-01-01 00:00:01 is 1,388,516,401 seconds after January 1 1970 00:00:00 UTC.

Can you try this,

echo date("Y-m-d H:i:s", 1388516401);

As noted by theGame,

This means that you pass in a string value for the time, and optionally a value for the current time, which is a UNIX timestamp. The value that is returned is an integer which is a UNIX timestamp.

echo strtotime("2014-01-01 00:00:01");

This will return into the value 1388516401, which is the UNIX timestamp for the date 2014-01-01. This can be confirmed using the date() function as like below:

echo date('Y-m-d', 1198148400); // echos 2014-01-01

I guess you are asking why is 1388516401 equal to 2014-01-01...?

There is an historical reason for that. There is a 32-bit integer variable, called time_t, that keeps the count of the time elapsed since 1970-01-01 00:00:00. Its value expresses time in seconds. This means that in 2014-01-01 00:00:01 time_t will be equal to 1388516401.

This leads us for sure to another interesting fact... In 2038-01-19 03:14:07 time_t will reach 2147485547, the maximum value for a 32-bit number. Ever heard about John Titor and the Year 2038 problem? :D