diff --git a/src/modules/clock.cpp b/src/modules/clock.cpp
index 36581e51..97777667 100644
--- a/src/modules/clock.cpp
+++ b/src/modules/clock.cpp
@@ -402,8 +402,13 @@ auto waybar::modules::Clock::get_calendar(const year_month_day& today, const yea
               data = g_utf8_find_next_char(data, end);
             }
           }
+          // Note: the stream's default fill character is already a space (L' ' on
+          // libstdc++'s wide FormatStream, ' ' on libc++'s narrow one), so no
+          // std::setfill is needed. Passing std::setfill(' ')/std::setfill(L' ')
+          // here is not portable because the fill char type must match the
+          // FormatStream's char type, which differs between standard libraries.
           os << Glib::ustring::format(
-              (cldWPos_ != WS::LEFT || line == 0) ? std::left : std::right, std::setfill(' '),
+              (cldWPos_ != WS::LEFT || line == 0) ? std::left : std::right,
               std::setw(cldMonColLen_ + ((line < 2) ? cldWnLen_ - wideCharCount : 0)),
               calendarLine);