If you want to add or subtract some time from DateTime value, SQL Server has very useful DATEADD function.
For example, to add two days to SomeDate field, use this code:
SELECT DATEADD(dd, 2, SomeDate) FROM SomeTable;
To subtract time, just use negative value of second parameter.
Syntax of DATEADD function is:
DATEADD( TimeIntervalName, Value, FieldOrVariable )
In previous example, d is abbreviation for day. Here is complete list of interval names and their abbreviations that can be used with DATEADD function:
Second: ss or s
Minute: mi or n
WeekDay: dw or w
Week: wk or ww
Day: dd or d
DayOfYear: dy or y
Month: mm or just m
Quarter: qq or q
Year: yy or yyyy
Note that Nanosecond and Microsecond can be used only on SQL Server 2008 or later.