This happens in both single and multiplayer, on occasion.
If the game is paused before clicking the ok box at the very beginning of a game (where it tells you the victory conditions) for a somewhat lengthly amount of time, IE: to do religious tolerances and missions and such, then when the game becomes unpaused it will forever remain January 1st, 1492, even if paused and unpaused again.
Furthermore, sometimes pausing can cause strange problems, particularly with diplomatic options, such as clicking the offer royal marriage button and nothing happening. This is probably related to the game requiring a day to pass, before something works, but because it's paused nothing happens.
If the game is paused before clicking the ok box at the very beginning of a game (where it tells you the victory conditions) for a somewhat lengthly amount of time, IE: to do religious tolerances and missions and such, then when the game becomes unpaused it will forever remain January 1st, 1492, even if paused and unpaused again.
Furthermore, sometimes pausing can cause strange problems, particularly with diplomatic options, such as clicking the offer royal marriage button and nothing happening. This is probably related to the game requiring a day to pass, before something works, but because it's paused nothing happens.
