This date is automatically generated, it can't be changed.

If you want a place where you can specify such a date, I'd suggest using custom fields (in Extra Fields or information tabs).

i am tech challenged, and thunderbird is brand new to me new to me [and to this computer]. how do i get it to automatically generate the correct date? i began entering info within the 4 days, and it has automatically entered 12/10/17, 02/10/2018, & 03/10/18. today [10/16/17] it enters 04/10/18. how do i enter today on some clock somewhere? my email comes in showing the correct date & time. i don't want to have to manually enter a date in an extra field each time i modify other data in the address book.

I would expect the 'LastModified Date' to use the computer date/time settings. Obviously everything was ok on the first day - 12/10/17

But you then have 02/10/2018, & 03/10/18 which probably should be 14/10/17 and 15/10/17 respectively.

It's showing 2018 and 18 as the years which makes me wonder if you (or anyone else who has access ) have been in the Control Panel of computer to alter settings as you would not have expected the year format to change on it's own; let alone the day and year as well.

Are you absolutely certain that you (or anyone) have not been in the Control Panel of computer and performed any modifying of 'Date and Time' or Timezones or 'Regional and Language Options' ?

If you open a document, notepad anything but not anything to do with Thunderbird and modify and save, is the modified date correct for that document?

Please list all your addon extensions whether they are enabled or not. In Thunderbird 'Help' > 'Troubleshooting Information' click on 'Copy text to clipboard' button paste in this forum question 'Post a Reply' text box. You can remove/edit all other info - just need everything under 'Extensions' section.

Test to rule out interference from other addons: 'Menu icon' > 'Addons' Disable all addon extensions except 'MorefunctionsForAddressBook' then restart Thunderbird modify a contact and check the modified date. Provide feedback on results.

Thinking out a loud here.... This looks like someone altered something in Date and Time /Time zone/Formatting of date etc and then maybe changed some settings back but not all etc etc. Sooo....I'm wondering if this could be caused by a temp file using an incorrect modified date which it got whilst time/date etc was not correct.

In Thunderbird 'Menu icon' > 'Tools' > 'clear recent history'

Close Thunderbird now

Clear all Windows cache files which you will find in this 'Temp' folder: C:\Users\<Windows user name>\AppData\Local\Temp

Access this file: C:\Users\<Windows user name>\AppData\Local\Thunderbird\Profiles\

It will contain your profile name folder usually 'xxxxxxxx.default' where x's are letters and numbers, but it's not the one which Thunderbird uses to contain all your profile accounts, address books, emails etc. Windows creates it and uses it as a cache. Access the 'profile name folder' 'xxxxxxxx.default' and clear all contents or if concerned move them to a folder on the desktop as a temp backup.

Restart Thunderbird. Modify a contact info and check results.

Please report back on results.