Errata patches for calendar have been released for OpenBSD 7.7 and
7.8.

Binary updates for the amd64, arm64 and i386 platform are available
via the syspatch utility.  Source code patches can be found on the
respective errata page:

  https://www.openbsd.org/errata77.html
  https://www.openbsd.org/errata78.html