Change your database.
Stop storing dates and times as NVARCHAR, and stop storing them separately. Always store dates and times together, so they form a single "moment in time" in DATETIME variables.
Then you can just use the
DATEDIFF function[
^] which returns what you want.
If you store them as strings, you have to validate and convert them every single time you want to use them, and that's bad. A single error that manages to get into your DB will mess up all your queries.