Save (or retrieve) the date I added a contact to Thunderbird

[Kodiologist]

I'd like to keep track of the date on which I add a contact to an address book on Thunderbird 52.1 (specifically, an address book that I keep synchronized with one on fruux.com using the Thunderbird extension Inverse SOGo Connector). Failing that, it would be nice to know the last modification date of a contact. Either would give me a sense of how old the contact information is.

If this information is already saved by Thunderbird, how can I retrieve it? If not, how can I cause it to be saved?

(Full disclosure: I also posted this question a week ago on superuser.com.)

[sfhowes]

The MoreFunctionsForAddressBook add-on gives the option to display the 'Last modified date' column in Address Book, e.g. a date shown as 1498276990 can be converted to the actual date Saturday, June 24, 2017 4:03:10 AM.

https://freeshell.de//~kaosmos/morecols-en.html

https://www.epochconverter.com/

[Kodiologist]

The use of the POSIX timestamp as the display format is inconvenient, but it works! Thanks.