LOF Glass Dating for Z28 Questions....

[RamAirBirds]

What is the Consensus on Dates for Glass on a car? How Far in advance of build date for Windshield / Back Glass / Side Glass
I have a LA Built 1968 Z28 with a Build Date of 06C that has what I consider to be original glass (except for windshield.)
The Side and Quarter Glass are dated IX (June,68) and the Rear Glass is XX (February, 68)
Could this be correct? I guess I am more surprised by the side and qtr glass being dated same month as cars build date.
I am getting ready to order glass (Auto City Diamond) for both a 69 and 70 T/A and am curious what the "experts" say about correct date ranges for glass....i.e. how much spread etc....
Thanks in advance for any and all comments...