Thanks ALL for your help I couldn't do this on my own.
What I ended up doing was in a Query.
2WeekNumber: Int(([MyDate]-#2013-12-28#)/14)
FirstDayIn2Weeks: DateAdd("d",[2WeekNumber]*14,"28/12/2013")
LastDayIn2Weeks: DateAdd("d",[2WeekNumber]*14+13,"28/12/2013")
I would have liked this in a...