engine and body build dates

Normally, how close are the body tag and engine build date codes?

From what I've seen over the years with GM cars, there's about a 2-3 week "lag" between engine date codes and chassis date codes. Sometimes more. In the early '70s, there was a strike at certain plants and date codes showed up from the previous year, since the line workers weren't the "regular" guys, and they took parts and pieces where ever they could find them. I suppose I mean there's no "hard/fast" rule for such things, but there are "generalities".

FWIW

Jim