From what I see here, it's pretty simple. Don't use a custom format
select the one under "Time" that matches the format you've listed. Al
you have to do is remember to actually type in the colon when you ente
the time and the format will remain static.
Is there a reason that you can't include the colon when entering th
data ? If there is, post the details and I'll try and figure somethin
out for you