After updating a record, some date/time fields are set to 1969-12-31. This date may be unexpected, but its appearance in a date/time field is not random.
Date/time fields in ServiceNow are stored internally in the database as a large number; it is the number of milliseconds between the base date/time of 1970-01-01 00:00:00 GMT and the date/time specified.
A large number of milliseconds is not user-friendly, so the instance converts it to the user's expected date/time format and timezone.
If a date/time field is null, it can be initialized and treated as a zero to avoid errors. Note that this results in the instance interpreting the value as the base date/time of 1970-01-01 00:00:00 GMT.
If you are in a different timezone that is -GMT, when the instance displays this base value in your timezone, it will be 1969-12-31, and the hour will be adjusted by the offset between your timezone and midnight GMT.