How To Combine Date And Time Into One Field

We have a piece of software and database for student registers.

One of the biggest problems is the database has been difficult to work with, as I have had to work out the date using dateadd functions and combine fields from different tables.

I've now got the date correctly, but I want to add the time to the time part of a datetime field.

The time field is already stored in a datetime field, but the date in this field is always 1899-12-30.

This is the function I am using to get the date:

Code:
DATEADD(dd, e.day_of_week, DATEADD(ww, sm.week, '01/08/2011'))

And this is the function I use to get the time (from a datetime field):

Code:
SUBSTRING(CAST(e.start_time AS varchar(20)), 13, 5)

With the DATEADD function it appears you can only add individual elements each time hh, mi, ss.

Would I have to add a DATEADD function for each time element in order to add the time to the date?