WinDat Opener does its best to preserve the "Last modification" timestamp on any attachments that it saves. However, the TNEF format, which was created in the infancy of e-mail, does not not have a consistent way to store timezone information. 

Because of this timezone issue, the timestamp that WinDat Opener gives a file may be out to the extent that the timezone in which it was created is different to the local timezone, depending on which e-mail client sent the item, and the behavior of the servers that processed it. 

There is no workaround for this issue; most TNEF decoders (including Microsoft's) do not preserve file timestamps at all, and simply use the current time.