You could just use an IF statement... something like (this is just for the concept...)
IF [date/time] < Apr-16 Then '12.15-03.16'
You may need to do some formatting and date manipulation using DATEPART() but should be easy enough.
Is that enough to get you started?
Happy to help further, although some example data would help.
It is great! Thank you Mark
Best regards from Berlin!
You're welcome, neighbor...!
Best regards from The Netherlands