That's been the normal trend that we've noticed in the converting of the saves from 1.52 to 1.61.
The one I sent you I realized I opened using the 1.52 that I still have on my PC in another folder, so you received it in 1.52. When it convertd to 1.61 the dates changed.
I've seen this already in the other games that I've loading from 1.52 into my 1.61 and sent on to the next player. It doesn't actually change what's happened, as far as we can tell. It's just changed the date. Though it looks like some of the games the date moves forward and some it moves backwards, since on of my games went from 1880 AD to 1775 AD.