On Fri, Nov 28, 2025 at 10:37:38AM +0000, Stuart Henderson wrote:
> taking a cue from libc's "support" for %E/%O (C99 locale modifiers
> which are recognised but ignored) here's a diff to ignore the modifiers
> so at least we'd get something sensible printed in these cases. (if
> testing with date(1) note that it's statically linked). would this
> or something like it make sense?

It's not much more complicated to actually implement the modifiers...

Here is a quick example diff to add them to %I in strftime.c which should fix
the OP's specific problem.

Is there any reason not to implement them for other conversion characters?

--- lib/libc/time/strftime.c.dist	Fri May 16 14:24:39 2025
+++ lib/libc/time/strftime.c	Fri Nov 28 13:46:25 2025
@@ -129,7 +129,11 @@
 static char *
 _fmt(const char *format, const struct tm *t, char *pt, const char *ptlim, int *warnp)
 {
+	char	fmt_mod;
+	char *	fmt;
+
 	for ( ; *format; ++format) {
+		fmt_mod = 0;
 		if (*format == '%') {
 label:
 			switch (*++format) {
@@ -211,9 +215,21 @@
 				pt = _conv(t->tm_hour, "%02d", pt, ptlim);
 				continue;
 			case 'I':
+				switch (fmt_mod) {
+				case '_':
+					fmt = "%2d";
+					break;
+				case '-':
+					fmt = "%d";
+					break;
+				case '0':
+				default:
+					fmt = "%02d";
+					break;
+				}
 				pt = _conv((t->tm_hour % 12) ?
 					(t->tm_hour % 12) : 12,
-					"%02d", pt, ptlim);
+					fmt, pt, ptlim);
 				continue;
 			case 'j':
 				pt = _conv(t->tm_yday + 1, "%03d", pt, ptlim);
@@ -477,6 +493,11 @@
 				pt = _fmt(Locale->date_fmt, t, pt, ptlim,
 					warnp);
 				continue;
+			case '0':
+			case '_':
+			case '-':
+				fmt_mod = *format;
+				goto label;
 			case '%':
 			/*
 			** X311J/88-090 (4.12.3.5): if conversion char is