Use either the "Date" or the "Time" datatype, depending on what you're looking for, to record the time. If you're only concerned with days, "Date" is better. Likewise, if you're after hours, minutes, and seconds, use "Time" (although, unless I'm mistaken, time will store the date as well).
Datediff uses 3 parameters: a string telling you what type of interval you want (i.e. yyyy for year to four places, q for quarter, h for hours, etc.); date 1; and date 2. You can do a web search for more info on how to use it.