Creation and modification rules can be defined by using either absolute or relative date formats. Absolute dates are dates that are displayed as real calendar dates, e.g. 2/19/2009. This format is always assumed and most widely used. Relative dates are dates that get interpreted as the time relative to the instant in which the rule is being evaluated. Specifying a date rule this way is advantageous when you want to synchronize files within a particular frame of time.
To specify a relative time, the format you must use is (+/-)DD:HH:MM:SS. Thus specifying "-180:00:00:00" would mean "180 days ago". Running this sync will include all files/folders which have been modified in the past 180 days (provided you use the "is after" selection). Use whatever number you want here to indicate the time frame desired.
Note: Don't forget to include the '+' or '-' sign!