In my computer with LANG set to en_US.utf8, en_GB.utf8 or fi_FI.utf8 yields;
? IsDate("2488329.65453707")
False
But with fr_FR.utf8 I got True.
I have no idea about date formats of France, but that seems to be error.
Revision 5250 @ Xubuntu 12.04 64bit
Jussi