Used in a situation where you
haven't done anything cool or unique, and your
day was as boring as ever; but it also can be used in awesome situations, like when your buddy Person 1 sees you parachuting off of the 2nd floor.
TL;DR: Synonymous with "same shit, different
day".
Person 1: So, how was your
day?
Person 2: Just another day at the office.
Sam: Jesus, they worked you over good. You all right?
Nathan: Just another day at the office.