Restrict entries according to date in another cell

My fairly simple mileage expense spreadsheet is submitted for reimbursement once each month. Cell B5 is a drop down list of months & the year (i.e November 2008) from a list on a separate sheet in the same workbook. This is used as the time frame for the travel charges being submitted. Cells in column "A" are supposed to contain the specific date within the month that the travel occurred. Because I'm such a lousy, slow typist I'd like to be able to just enter one or two digits for the specific day and have Excel extract the month abbreviated to 3 letters (i.e. Nov) using the information in the "period" cell B5. My fingers sometimes work faster than my brain and I get letters/numbers jumbled so it'd also be nice if there was some error checking that would disallow entering a 31 if the month listed in cell B5 only has 28 days. Possibly a calendar lookup?? I've searched enough and come across a lot of words/names but get bogged down in the syntax/structure.

Probably like everybody else coming to this forum, I've whiled away many an hour entering formulas in a variety of ways and searching the internet and I'm as frustrated as everybody probably is/was. And, also like most other things, it's probably not difficult if you know what you're doing. I obviously don't but it's fun in a masochistic kind of way.

Attached is the form and thanks for whatever help you give.
Ross