Found the following significant warnings:
  /home/hornik/tmp/R.check/r-devel-clang/Work/build/Packages/tzdb/include/date/date.h:966:34: warning: identifier '_d' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  /home/hornik/tmp/R.check/r-devel-clang/Work/build/Packages/tzdb/include/date/date.h:967:34: warning: identifier '_y' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  /home/hornik/tmp/R.check/r-devel-clang/Work/build/Packages/tzdb/include/date/date.h:1975:13: warning: identifier '_d' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  /home/hornik/tmp/R.check/r-devel-clang/Work/build/Packages/tzdb/include/date/date.h:1983:13: warning: identifier '_y' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  /home/hornik/tmp/R.check/r-devel-clang/Work/build/Packages/tzdb/include/date/iso_week.h:750:13: warning: identifier '_y' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  /home/hornik/tmp/R.check/r-devel-clang/Work/build/Packages/tzdb/include/date/iso_week.h:758:13: warning: identifier '_w' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  ./ordinal.h:240:40: warning: identifier '_yd' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  ./ordinal.h:241:40: warning: identifier '_y' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  ./ordinal.h:631:13: warning: identifier '_yd' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  ./ordinal.h:639:13: warning: identifier '_y' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  ./week.h:880:13: warning: identifier '_w' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
See ‘/home/hornik/tmp/R.check/r-devel-clang/Work/PKGS/clock.Rcheck/00install.out’ for details.
* used C++ compiler: ‘Debian clang version 19.1.7 (1+b1)’