I was in the process of making a method, and decided to investigate the effects of feeding meat. The items value chart shows it adding 1 week of lifespan. I took the Golem that MR2 makes when placed in MR1, and raised it several ways.

All Rest, fed Fish, no items: at 2y3mo3wks. got the "Most Important Stage" warning

All Rest, fed Meat, no items: at 3y1mo. got the "Most Important Stage" warning.

Essentially, it looks like meat sets the clock back one week!

Here's the math:

We started with a monster who gets the "Most Important" warning at 2y3mo3wks*. (111 weeks) and fed meat, so that meant it got 28 feedings therefore 28 weeks added (or reset):
111+28
But wait! I fed meat during those 28 weeks, 7 times:
111+28+7
-And, of course, I fed meat during those seven weeks, turned out to be twice, based on the week it was born:
111+28+7+2=148 or 3y1mo.
QED

*A Monster Rancher year = 48 weeks