Custom fields of type date can be defaulted to either specific or relative dates. Typically, relative dates is the scenario that makes sense in most of the cases.

The format for specific dates is an integer which indicates the number of seconds since the Unix Epoch (January 1 1970 00:00:00 UTC), which is the format consumed by the PHP date() method.

The relative scenario expects default values like {tomorrow}, {yesterday}, {+2 days}, {-3 days}, {next week}, etc. The curly brackets indicate that this is a logical value which is then evaluated using the PHP strtotime() function.