I don't know if this will work for you since it depends on the format of the values in your database, but you could try this advanced trick.
Let's assume your date field is called MY_DATE. Go to Database > Select Table, and double-click Custom. The custom query will initially look like "SELECT * FROM ...". Don't change whatever comes after FROM, but edit the SELECT part so that it reads:
SELECT *, DateAdd("d", 50, MY_DATE) AS DATE_PLUS_50 FROM ...
This custom query allows you to access the original fields in your database plus a new field called DATE_PLUS_50. The value of this new field can be formatted in an expression using PSM's DATE function.