Three possible options quickly come to mind:
1) I would suggest to put a front end data filter to strip those out in your Tableau data source.
2) Or you could try duplicating the date field and add it as a second date filter to exclude the 9999 and add it to context. I haven't tried this but I would hope the latest date filter would then follow this context date filter.
3) Change your date to 1900 ;-)