Both the AddHours and AddMinutes methods work fine - they are old methods that have worked for years, to it's very unlikely t5hat a total failure of that form would happen.
But I'd still use a Timespan, as I suggested yesterday when you asked a very similar question:
How do I best update a datetime variable from two separate time/date edit boxes?[
^]
And I'd use the debugger to find out exactly what values I was working with instead of assuming it's a .NET Framework error!