Je is an abstract preposition. One of its frequent uses is for times and dates:
Tio okazos je la deka horo. That will happen at ten o' clock.
Tio okazos je la deka de Oktobro. That will happen on the tenth of October.
Notice that dum and en won't work for "o' clock", as dum la unua horo means "during the first hour", i.e. before one o' clock. In contrast, there is no problem with saying dum/en la deka de Oktobro.
Therefore, in the absence of a noun (and without context clues) I would interpret je la deka as "at ten o' clock" rather than "on the tenth".
Tio okazos je la deka. That will happen at ten.
Tio okazos la dekan. That will happen on the tenth.
It is conventional to keep the accusative for numbered days if there is no noun. Be aware however that these are conventions rather than rules. You may also encounter sur or ĉe for either.