Comment by SebastianKra

Despite it's steep price and Mac-exclusivity, I've found https://timingapp.com to be invaluable.

It logs open applications, documents, meetings, calendar events and recently even iPhone apps. You can then, manually or automatically, create time-blocks from this data.

I usually use it at the end of the day, but I once was able to recreate a rough timesheet for the last three months in 10 minutes.