When created or editing a topic, if the option for topic type is:

• either Sticky or Announcement, entering a non-zero number of days for Stick topic for: is stored in the database and has effect.
• Global, entering a non-zero number of days for Stick topic for: is ignored (no warning or error) and zero is stored in the database, rendering a Global topic type as never ending.

I believe that Global Announcements time limits should act the same as Announcements and Stickies?

See https://www.phpbb.com/community/viewtopic.php?f=556&t=2415036 for discussion of the issue.