The date constants/literals make sense only in the context of a query.
If by "in memory" you mean Apex code, then something like this is needed:
digest.DM_Digest_date__c = System.today() - 7;
digest.DM_Digest_date__c = Date.newInstance(2012,12,31);
You can read up more about Date class's methods (plus System class has
now() methods too).
Not to mention the fact that
LAST_N_DAYS etc all cover timespans, not actual points in time... They're really just a way to quickly write
WHERE CreatedDate > :startPoint AND CreatedDate < :endPoint. So you can't really assign this to one date/time variable, you'd need 2 fields if you want to record a date "from-to".