Something similar is the odometer example given in this KB article:
Referring to a Field in the Previous Record or Next Record http://support.microsoft.com/?id=210504
Also, check out this thread and try downloading the sample that Max mentions
in the first message:
If this is for a homework assignment... read your assignment.
If this is for a home-built checking account monitor database...
Microsoft Money, Quicken (though I really can't recommend it now that
Intuit's screwed it up so badly), and dozens of inexpnsive, shareware,
or freeware checking account programs are available. It makes NO sense
to reinvent one.