This isn't my best area, but two thoughts come to mind:
1) Indexing. Are there indexes built on this timestamp field? If this is a large SQL table, and there are no indexes, we might be hitting a timeout issue.
2) Timestamp format. Depending on the format the SQL Server database is using, FileMaker might be choking on the format in which the timestamp is stored. (This is an idea I'm throwing out; I don't have a good solution for it.)
Just a couple of ideas to think about.