Model Name : Vigor2850n
Firmware Version : 3.6.8.5_2471201
Build Date/Time : Aug 5 2016 10:43:04

After a reboot, the email ejected by the notification system indicating the WAN/Router is up is dated 1/1/2000 (or thereabouts).
This can means it's not shown as a "recent" email in the email client (in my case Thunderbird).
i.e. It's at the very bottom of the list of emails and so it might not be noticed.

Suggestion:

On a reboot request, store the current time and date.

Then on restart:

If NTP configured but unavailable, use the date & time stored just prior to the reboot.
Use the previously stored date & time for the ejected email timestamp.
Include a note in the email - "No time & date update available from NTP - Router reboot requested at dd/mm/yyyy - hhmm"

OR

If NTP not configured and if the date & time not updated via the browser, use the date & time stored just prior to the reboot.
Use the previously stored date & time for the ejected email timestamp.
Include a note in the email - "No time & date update available from Browser -Router reboot requested dd/mm/yyyy - hhmm"

OR

If the time is updated by NTP or the broswer - use the (updated) router data and time.

EDIT: ...Or put a RTC in the router...