Combine date and time columns

Here's one that has been driving me nuts! I have several very large files that have the date (01/01/01)
in one column and the time (14:30:25)in another.
I need some way of combining these two values into
one column so that the output will be recognized as
a date/time format (ex. 01/01/01 14:30:00) for
graphing purposes. Using the ampersand seems to code
it as text and I just get out a number (ex. 36892)
that is related to the date, but I cannot get Excel
to format the new cell with the proper date/time
readout. Any suggestions????

Thanx, Tracey