Apply hour offset to values

[cseymr]

Related to #7 - in one implementation, it was important for me to make sure that the Date was rendered correctly irrespective of the user's local timezone, which was achieved by adding 12 hours to the datetime (this worked for anyone except users in Tonga, I think, but there were no users based there :-) )

So a utility which allowed one to add/remove a fixed number of hours to a DateTime field would be useful to me personally - it's probably not a widely required feature though :-)