I tried the valueFormat "YYYY-MM-DD[T]HH:mm:ss.SSS[Z]" which works in that it uses the "Z" in the saved value like this "2019-09-20T05:15:00.000Z". But when the ".SSS" is removed it does not work as you have noted (using "YYYY-MM-DD[T]HH:mm:ss[Z]" for valueFormat results in "2019-09-20T05:15:00.000-07:00").
If you can accept the milliseconds in the format, this may work you.