Sure, the in-game date must be written in the savegame meaning it can be changed. But you need to be in port otherwise the game will not like it...
For not messing everytime with the savegames you can use SH3 Commander to set exactly how many days you want to be in the bunker. Maybe try to set a time backwards with it to see if it works, you need to use its config files if the menu doesn't allow for negative values, I don't think it works though and you need to edit the saves directly.
For realism, I don't know, I don't care too much so I hope others will jump in here. Besides: changing date is not realistic
