In numerous places, the law refers to: "after Sept. 13, 1994...blah blah blah.
In my opinion, the ban sunsets after Sept. 13, 2004, i.e. Sept. 14. Also, if you are a betting man and there is this much confusion out there, do your business on the 14th. No need to be busted for a technical violation by being a day early. Not that anybody is watching on that day, but if you buy an LEO rifle on the 13th and it comes into question later, you might have a problem when the prosecution argues the fine line of dates and +/- midnight.
923(i): ..."The serial number of any semiautomatic asault weapon manufactured after the date of the enactment of this sentence shall clearly show the date on which the weapon was manufactured"..."A large capacity ammunition feeding device manufactured after the date of the enactment of this sentence shall be identified by a serial number that clearly shows the device was manufactured or imported after the effective date of this subsection,..."
178.92 (a)(2):"...In the case of any semiautomatic assault weapon manufactured after September 13, 1994..."
178.92 (c): "Large capacity ammunition feeding devices manufactured after September 13, 1994."
etc. etc.
Also notice that LE mags from Glock and SIG are all date stamped 9/14/94.