The poster says six, the group message says half past six, and the shared calendar still shows last month’s room. A calendar works better when somebody knows which record is authoritative.
Choose the record that gets updated first
Agree where confirmed details live and who maintains them. Other notices can link to that entry and display when their own information was copied. The arrangement need not require new software; a maintained event page may be enough.
Give corrections a route
Place a contact beside the listing for wrong times, inaccessible links or venue changes. When a correction arrives, update the main entry and then identify the places that copied it. Editing only the newest social post leaves older versions circulating.
For the next event, make a tiny distribution list: calendar, poster, email and group message. Tick each after the details have been checked. This is especially helpful for repeating events, where a familiar title can hide a changed date. A clear correction routine matters more than a calendar full of attractive but inconsistent entries.