The Unix epoch starts at 1 January 1970. The value of Unix time number is zero at the Unix epoch, and is increases by exactly 86,400 per day since the epoch. Because Unix time is based on the Unix epoch, it is sometimes referred to as "epoch time".
Leap seconds are the time in seconds added to the UTC to keep the system clock in sync with mean solar time
Unix epoch cannot represent leap seconds. During leap second, the UTC day is not exactly 86,400 seconds long. So the OS has to slow down the clock to accomadate the leap second. Slowing down of OS clock can be achieved by repeating the 60th seconds.Hence the unix epoch for a day is always 86,400 seconds.
Unix epoch is caluculated as a signed 32-bit interger and has a maximum value of 2,147,483,647. This maximum value reaches on 03:14:07 UTC on 19 January 2038 and after which the unix epoch cannot encode time