The Last Error Date verb will return an AppleScript date object containing the date and time of the last error that caused the last error or last error message to update. If no error has occurred since the addition of this verb it will return the current date so as not to cause scripting errors during the addition of the verb to the database.
The value of the last error date is preserved across restarts of XTension by being saved to the database and can be written to the log or used at any time to discover when the last script error occurred assuming that another script error has not occurred in the meantime that causes it to be overwritten.