What time do NOS IFR Charts expire?
In FAA-land the AIRAC cycle effective time is 0901Z. Your old charts were valid until 13 September 2018, 0900Z, the new ones are valid since 13 September 2018 0901Z. 

I can't find the regulation that says it's always the case, but Google FAA 0901Z and you see that will give a ton of new charts / publications. 

Here is a Jeppesen publication that explains the interpretation of the effective date.