It has been a bit difficult figuring out what you need to do. Sometimes it
helps to describe in non-database terms the result you are seeking. For
instance, "default" has a specific meaning that you apparently did not
intend.
What I suggested was the Format property. You need to enter the characters
exactly as I have shown you in the Format property. There is also a
property called Input Mask, which controls how the data are entered. You
could use something like:
00:00\ >??;0;_
This means the four-digit time is required (e.g. 07:30). The ">" converts
the letters (AM or PM) to uppercase. The question marks mean "AM" or "PM"
are optional. If you leave them out, Access assumes a 24-hour clock, so
07:30 defaults to AM, and 12:30 defaults to PM. 00:30 is 12:30 AM.
You do not need to type the colon. If you type 0730, you are entering 7:30
AM. 1230a becomes 12:30 AM; 0130p is 1:30 PM.
The 0 after the semi-colon means the display characters are stored; a 1
means they are not. Since this is happening in a Date/Time field I don't
think it matters which you use.
The underscore at the end of the Input Mask means the user sees _ as a
placeholder for each character.
You could use 99:00\ >??;0;_ or 90:00\ >??;0;_as the input mask. The 9
characters mean entry is optional, but this mean the user needs to use the
arrow key or the mouse to get to the second character when entering 730 for
7:30 AM, so you may as well use the first example.
There is more information in Help (although it seems to lack the information
about the semi-colons in the mask, at least in Access 2003).