This feature can be toggled and configured under Options -> Preferences -> General.
JabRef can automatically set a field to contain the date an entry was added to the database.
The formatting of the time stamp is determined by a string containing designator words that indicate the position of the various parts of the date.
These are some of the available designator letters (examples are given in parentheses for Wednesday 14th of September 2005 at 5.45 PM):