Mac High Sierra not able to open MacMail, iCal for more than 5 seconds
As long as your clock is displaying the correct local time, resetting NVRAM won't affect Mail or Calendar. I'd suggest being more aggressive in cleaning out of Mail's database files and then re-testing:


delete Mail.app's (~/Library/Mail/) data folder (not just its contents)
re-delete both /Library/Preferences/com.apple.mail.plist &amp; ~/Library/Preferences/com.apple.mail.plist
re-delete ~/Library/Preferences/com.apple.mail-shared.plist (if it was re-created)
re-test