IIRC (we don't have Selcopy at this site) you don't decalre fields as numeric or otherwise. so if you do a compare it will simply compare it as text data. As long as you've got numeric values in your file to compare with your constant date then it shouldn't be an issue.
If you do get non-numerics in your file, that's a different issue. It will consider "A" to be less than "1" for example, rather than throw any sort of error. But you could use a LT and a GT comparison to overcome that.
Bear in mind also, that in your example, you have the date in DDMMYYYY format, but to Selcopy it's just a string of numbers. So it will consider 01052010 to be less than 02052009, for example.
There may be date routines in Selcopy, although I've never used them. If you e-mail the guys at CBL with any queries, I've always found them to be extremely helpful.