File History date & time is wrong

Just gone to restore an earlier version of a commonly edited Excel file and noticed the file history date & time are wrong. Date is 18th June 2013 and this file was edited within the last 15 minutes:

However, file history shows a completed different date & time, even for the current version:

I’ve checked the time on the Linux server and that’s spot on. I’ve just created a test document and the file history is correct, i.e. it shows today’s time and date.

So it only appears to be occurring with existing/long standing documents. Will keep an eye on this.

I’ve checked other frequently edited Excel workbooks, e.g. my timesheet and invoices. They’re all showing the wrong dates in file history. Files created from scratch show the correct date.