Assert for years < 1000 needed

Hi,

from the mission preconditions years could be "0 < year <= 3000". But until now years between 0 and 1000 were not tested. Therefore I have seen solutions that will not work if the time sting doesn't include a year 4 characters long.

I have just added that test at Github. Please check and merge if this makes sense.

Thanks and best Regards,

Selindian