How to combine date and time to datetime in SQL Server?
The time part is the fractional part and the date part the integer part, so if you date is genuinely just a date (no time part at all) and your time is genuinely just a time (no date part at all) you can simply combine them by adding them together.

For example:

SELECT CAST(40597 AS DATETIME), CAST(0.5 AS DATETIME)
SELECT CAST(40597 AS DATETIME) + CAST(0.5 AS DATETIME)


The top SELECT shows what values represent today and 12 noon, and the second line adds them to make a single value representing today at 12 noon.