It is unfortunately not caused by ships exceeding their crew deployment times. This is an error message (see the 6.10 bug list thread where I reported it) that was confirmed by Steve to be due to overflowing the number a variable can hold.
From Steve for another person with the same error:
The problem is that I am using a long variable instead of a double to store the last launch time (which is stored for all ships even if they never enter a hangar). Because your game has been running for a long time, the number of seconds passed the game has exceeded the capacity of the database field. This is fixed for v6.20 but will continue to affect current games.
If anyone wants to fix this for a current game and has Access, you need to change the data type of the LastLaunchTime in the Ship table to Double (or currency).
The trouble is a long variable should not overflow until 69 years of service life as clement points out...and none of the ships I have are anywhere near that old.