Convert to and from Unix Timestamp
I think that the two most used libraries are the Adafruit RTClib and the pjrc.com TimeLib.
They both have functions to convert the epoch, and they both are reliable. But both lack the timezone and the DaylightSavingTime.
I think you find the pjrc.com TimeLib just slightly more suitable to handle the epoch time (which is a 32-bit unsigned long defined as 'time_t').