From Cumulus Wiki

Cumulus View menu.jpgUsed to hold the information that feeds the Webtags#Yearly for the 'thisyearT.htm' web template.

The diagnostic logs in the 'diags' sub-folder record before and after values for updates to highs and lows for monthly and annual extreme records, and can help if this file is corrupted by a false extreme. The stored values can be corrected (in Cumulus 1, there is no editor in Cumulus MX) using the This year's records screen on the Edit menu. Date, Time and Decimal formatting is as per your regional settings on the PC running Cumulus. If you cannot find the file see this FAQ.

These lows and highs and their timestamps can be seen by picking the Highs and Lows - This year screen from the View menu (Cumulus 1 only - see picture).

Like alltime.ini this log file has section headings with lists of properties (attribute = value). For more information on this file see in the Cumulus help file, in the section “Data log file format”.