You may want to entertain the question: what is it that you are trying to achieve - as in the final result? Then consider this SAS DOC reference, with relevance to external date formats:
http://support.sas.com/documentation/cdl/en/acpcref/61891/HTML/default/a003079774.htm
So, if you have an alternate baseline where 0 represents some particular point in time (date or date/time), then my suggestion is that you decrement from SAS DATE (days since 1/1/1960) or DATETIME (seconds since 1/1/1960) back to your baseline DATE/DATETIME representation, using the quite-useful MDY and/or the DHMS functions -- let SAS do the calculations for you rather than attempting to handle the calculation, instead.
Scott Barry
SBBWorks, Inc.