4 tips:

1) Note datetime is a float and a hour is a diference by 4.1666666671517305E-2 u can use these values to make the quick calculations from -4.5 hour or +1 hour o 15 minutes! Less  processing at functions is good for tunning.

2) Read a good article about diference in datetimes formats around the world http://www.sqlservercentral.com/columnists/dasanka/globilizationinsqlserver.asp

3) Store all data at the UTC (or UMC) is good. You can also store the one local datetime at another colummn or just the diference with a LOCAL_datetime-UTC_datetime.

4) U can make all at UTC and let the developers concern about show the correct local datetime and datetime format at the user's interface layer. Argue its cheaper to DB maintenace and good for globalization (yeah it´s right!).

Jean