This really sounds very trivial. Are you sure that you don't have any bugs in the functions which converts the datatime values into text and back?

>The dates and times are stored as character data, such as "06/10/2008-06:00". I didn't build this system from scratch, but inherited it a few years ago.
>
>>I haven't studied your code, but how is the time stored? If you use datetime fields, this should be extremely easy.