Devon,
This can be somewhat database dependent. I don't know how the standard SQL Server handles date addition. However,most have it. One which I uses has the following:
SECONDS( datetime-expr, integer-expr ) Add integer-expr seconds to the given date/time. If the integer-expr is negative, the appropriate number of seconds are subtracted from the date/time.
Since you know the start date, this would easily calculate the date you are wanting.
Hope this helps.
Russ