The description of Time/Date controls in Default Sub Settings is wrong
In "Setting default subscription options for group members", p 18-20, each of Timezones, Time Display, Date Display and Monday Start ends with the note:
Note: A change to this setting applies only to new members of the group; a change here does not affect existing members.That text actually applies to all of the other settings on this tab of the Settings page, not to those. This note should be placed prominantly at the beginning of the section, not duplicated for each of the non-time settings.
Timezones, Time Display, Date Display and Monday Start apply only to new Users of Groups.io - people whose email address, prior to joining this particular group, had not joined another group or had otherwise set those controls in their Account settings.
The placement of these settings was topic #71740860 recently in Beta, and their exact operation has been a FAQ in GMF for some time.
(And yet, somehow, I'm not finding it in our Wiki. Gotta go fix that.)