The web client (and others, perhaps; I haven't tried them) does not seem to like dates entered with "sept" as an abbreviation for September. It always assigns some wonky date (e.g. "sept 24" --> "Wed Sep 02 09 at 12:00 AM").

This has been a problem for over three years. C.f. http://www.rememberthemilk.com/forums/help/1093/#forumpost

Seems like a pretty standard way of entering a date to me. Let me know if "sept" works for others...