Date/Time fields come in three varieties that correspond to the three types of date/time fields in Relate Script: Date, Time and DateTime. Remember that these are separate and distinct types. Changing the field from one type to another will probably cause any formulas using the field to break. The final important detail is this: DateTime values will be represented in the current user's preferred time zone. You can, of course, easily convert them to another time zone, but be careful not to depend on a particular time zone being the default representation.