Timestamp does not change

I work with autocad lite 98 on a NT4 workstation. The drawing files are on a Nt4 server NTFS partition. After changing and saving drawings the timestamp does not change. The changes in the drawing are OK. Saving on the local disk the timestamp is OK. saving a little textfile on the server in the drawing folder ist OK too. I think its a windows problem or combination of workstation/server/autocad. Some days ago everything was OK. I need timestamps to make icremental backups with xcopy.