Problem with handling dates

hi all
I have a stored procedure that gets date from a particular field using the DATEPART function. However, somewhere in between it has to do an update which is why it adds hours, days or months to the function. Here is the code
SELECT @.var_NewNextRunDate=(CAST(DATEPART(YYYY,@.DT) AS CHAR(4)) + '/'
+ RIGHT(CAST(100+DATEPART(MM,@.DT) AS CHAR(3)),2) + '/'
+ RIGHT(CAST(100+DATEPART(DD,@.DT) AS CHAR(3)),2) + ' '
+ RIGHT(CAST(100+DATEPART(HH,@.DT) + @.var_Frequency AS CHAR(3)),2) + ':'
+ RIGHT(CAST(DATEPART(MI,@.DT) AS CHAR(2)),2) + ':'
+ RIGHT(CAST(DATEPART(SS,@.DT) AS CHAR(2)),2)+ ':'
+ RIGHT(CAST(DATEPART(MS,@.DT) AS CHAR(3)),3) )
Now the problem is that when there a change in day the hours get stuck because it simply adds the hours so if it is 23:40 and it adds 3 it becomes 26:40 which makes no sense. Can someone please help me find a solution to this? I have the same situation with days, and months as well.
Thank you.you should do all data/time manipulations on a date/time datatypes not chars.
if you are extracting a portion of an existing date/time which results in a char datatype, you should cast it back to date/time before you start manipulating it.|||Although this is an old thread I just thought I'd add what I did. I just used the dateadd function to resolve the issue. It worked just perfect.