How do I combine a date field with a string field of the time into one datetime field?

I am currently trying to convert a string field with the time stored in it (120000 = Noon) into a string that I can insert into the formula to create a datetime field:

{@Time} = LEFT(CSTR(Time of day field,0,""),2) & ", " & MID(CSTR(Time of day field,0,"") & ", " & RIGHT(CSTR(Time of day field,0,""),2)

DATETIME(Date field & {@Time})

I receive the error message bad date-time format string.  Any suggestions would be appreciated.