Primary Key - datetime type

[Mike Cornelison]

The purpose of my db is to log reservations. For any given day, there are only 7 times available that can be reserved.

Because only one reservation can be made for any given time, I figured the best way to layout my table would be to set a datetime type as the primary key.

Datetime is an 8 byte data type, however, and I could simplify the entire time part to nothing more than 8, 10, 12, 14, 16, 18, 20.

Should I consider going to a fixed-length, 13 character string using the format of

YYYY-MM-DD ##

or should I stick with the datetime data type?

I think the most important aspect would be to consider that every time a customer clicks a date on the calendar, we'll be searching through the entire database of reservations to see if any reservations have been made for that exact date, is there a possible performance difference that could lead me to one data type over the other?