SQL Server 2012 :: Using CAST Or Convert Functions

How do I use the CAST or CONVERT function in the code below, I require a third column (named Diff) which Minus the StartTime from the EndTime and the result is outputted in the third column (named Diff).

Calculation: @Diff = (@EndTime - @StartTime)

I still want the variables (@StartTime and @EndTime) to remain as nvarchar.

The code:

DECLARE @StartTime nvarchar(10) = '12:10';
DECLARE @EndTime nvarchar(10) = '12:30';
DECLARE @Diff time(1) = '00:00';
SELECT @StartTime AS '@StartTime', @EndTime AS '@EndTimes', @Diff AS '@Diff';