The simple answer is: never retrieve the time twice if you need to use it again: it may well change!
So store it - or even store the filename - as I did in my answer to your previous question:
string fileName = @"D:\MyFolder\" + DateTime.Now.ToString("yyyyMMddHHmmss") + " Dogs.xml";
You then know you are accessing the same file.
If you are doing this in different methods, and you can't pass it from one to the other, then find the latest version - it's just a change on the code I gave you before:
string latest = Directory.GetFiles(@"D:\MyFolder", "* Dogs.xml")
.Select(s => new FileInfo(s))
.OrderByDescending(fi => fi.CreationTime)
.Select(fi => fi.FullName)
.FirstOrDefault();