Additional comment....
It is feasible to compress a time stamp to 4 bytes in a manner that takes simpler calculations than the scheme of converting the whole YY/MM/DD HH/MM/SS value to seconds.

Instead convert the YY/MM/DD value to a "days since reference". Two bytes easily handles well over 100 years. Then convert the HH/MM/SS to a value of 2-second periods since the beginning of the day. Two bytes can then keep track of the time in the day at 2 second resolution (which is most likely more than enough resolution for the data logger type application).

Michael Karas