You shouldn't be storing times as nvarchar. SQL Server has a time datatype (and plenty others datetime types).

We also have the function DATEDIFF available so we don't have to reinvent the wheel to do time calculations. DATEDIFF can implicitly convert well formatted strings into an adequate data type. Just be sure to have the correct values or you'll find errors.

Check the following example:

DECLARE @Sample TABLE(

OTIM nvarchar(8),

ReportedTime nvarchar(5))

INSERT INTO @Sample

VALUES

( '12:34:34', '12:34'),

( '12:34:34', '13:34'),

( '12:34:34', '11:34'),

( '12:34:34', '12:00'),

( '12:34:34', '1:00')

SELECT *,

CAST(OTIM AS TIME) OTIM_As_Time,

CAST(ReportedTime AS TIME) ReportedTime_As_Time,

DATEDIFF(HH, OTIM, ReportedTime) Difference_In_Hours

FROM @Sample