We had a similar situation at our club years ago, and for a season (or a series... I forget) what we did was back calculate a bunch of results to the same corrected time and assigned the resultant average rating to everyone.
It worked alright, but for some reason everything reverted soon after.....
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"