Please find below a patch to add an option (-h) to cal to allow
highlighting the current day.

For testing purposes, the -h flag takes an optional argument specifying
the date to highlight, e.g., cal -h2026-10-10 oct. I intend to remove
this before committing if the patch is accepted.

The highlight_day function was mostly taken from FreeBSD[1]. Their
implementation falls back to underlining the date (IIUC) if the terminal
has no standout mode. I decided to leave this out for now and simply
ignore the -h flag in this case.

To test this and check for regressions, I used the attached script. The
script tests the in-tree patched cal against the unpatched installed
one. It probably doesn't cover all scenarios, but hopefully enough.  In
fact, it is probably a little excessive, as it tests every day of 28
years, hehe. There's probably a smarter way to do this.

Lastly, I'm not super confident in my C skills yet so please don't hold
back on your feedback. I've added some comments with questions in parts
I'm not so sure about. I'd be grateful for any input on these.

[1] https://cgit.freebsd.org/src/tree/usr.bin/ncal/ncal.c#n1118

Index: Makefile
===================================================================
RCS file: /cvs/src/usr.bin/cal/Makefile,v
diff -u -p -r1.3 Makefile
--- Makefile	21 Sep 1997 11:48:29 -0000	1.3
+++ Makefile	3 Jul 2026 15:22:08 -0000
@@ -1,5 +1,6 @@
 #	$OpenBSD: Makefile,v 1.3 1997/09/21 11:48:29 deraadt Exp $
 
-PROG=	cal
+PROG=		cal
+LDADD+=		-lcurses
 
 .include <bsd.prog.mk>
Index: cal.1
===================================================================
RCS file: /cvs/src/usr.bin/cal/cal.1,v
diff -u -p -r1.34 cal.1
--- cal.1	2 Jul 2026 20:40:53 -0000	1.34
+++ cal.1	3 Jul 2026 15:22:08 -0000
@@ -41,7 +41,7 @@
 .Nd displays a calendar
 .Sh SYNOPSIS
 .Nm cal
-.Op Fl jmwy
+.Op Fl hjmwy
 .Op Ar month
 .Op Ar year
 .Sh DESCRIPTION
@@ -51,6 +51,8 @@ Calendars may be displayed by month or b
 .Pp
 The options are as follows:
 .Bl -tag -width Ds
+.It Fl h
+Highlight the current day.
 .It Fl j
 Display Julian dates (days one-based, numbered from January 1).
 The options
Index: cal.c
===================================================================
RCS file: /cvs/src/usr.bin/cal/cal.c,v
diff -u -p -r1.36 cal.c
--- cal.c	2 Jul 2026 20:40:53 -0000	1.36
+++ cal.c	3 Jul 2026 15:22:08 -0000
@@ -42,6 +42,8 @@
 #include <string.h>
 #include <time.h>
 #include <unistd.h>
+#include <curses.h>
+#include <term.h>
 
 #define	THURSDAY		4		/* for reformation */
 #define	SATURDAY		6		/* 1 Jan 1 was a Saturday */
@@ -127,8 +129,11 @@ const char	*day_headings = NULL;
 int julian;
 int mflag = 0;
 int wflag = 0;
+int highlight = 0;
+struct tm *local_time;
 
 void	ascii_day(char *, int);
+void	highlight_day(char *, int, int *);
 void	center(const char *, int, int);
 void	day_array(int, int, int *);
 int	day_in_week(int, int, int);
@@ -141,21 +146,26 @@ void	trim_trailing_spaces(char *);
 void	usage(void);
 void	yearly(int);
 int	parsemonth(const char *);
+int	is_today(int, int, int);
 
 int
 main(int argc, char *argv[])
 {
-	struct tm *local_time;
 	time_t now;
 	int ch, month, year, yflag;
 	const char *errstr;
+	const char *hl_date = NULL; /* debug only */
 
-	if (pledge("stdio", NULL) == -1)
+	if (pledge("stdio rpath tty", NULL) == -1)
 		err(1, "pledge");
 
 	yflag = year = 0;
-	while ((ch = getopt(argc, argv, "jmwy")) != -1)
+	while ((ch = getopt(argc, argv, "h::jmwy")) != -1)
 		switch(ch) {
+		case 'h':
+			hl_date = optarg; /* debug only */
+			highlight = isatty(STDOUT_FILENO);
+			break;
 		case 'j':
 			julian = 1;
 			break;
@@ -190,6 +200,13 @@ main(int argc, char *argv[])
 		day_headings = DAY_HEADINGS_JS;
 	}
 
+	(void)time(&now);
+	local_time = localtime(&now);
+	if (local_time == NULL) {
+		perror("localtime");
+		exit(1);
+	}
+
 	month = 0;
 	switch(argc) {
 	case 2:
@@ -202,8 +219,6 @@ main(int argc, char *argv[])
 			if (yflag)
 				errx(1, "specifying a month conflicts with -y");
 			month = parsemonth(*argv);
-			(void)time(&now);
-			local_time = localtime(&now);
 			year = local_time->tm_year + 1900;
 		} else {
 			year = strtonum(*argv, 1, 9999, &errstr);
@@ -212,8 +227,6 @@ main(int argc, char *argv[])
 		}
 		break;
 	case 0:
-		(void)time(&now);
-		local_time = localtime(&now);
 		year = local_time->tm_year + 1900;
 		if (!yflag)
 			month = local_time->tm_mon + 1;
@@ -222,6 +235,28 @@ main(int argc, char *argv[])
 		usage();
 	}
 
+	/* debug only */
+	if (hl_date != NULL) {
+		if (sscanf(hl_date, "%4d-%2d-%2d", &local_time->tm_year,
+		    &local_time->tm_mon, &local_time->tm_mday) != 3 ||
+		    strlen(hl_date) != 10)
+			errx(1, "illegal date format: use yyyy-mm-dd");
+
+		if (local_time->tm_year < 1 || local_time->tm_mon < 1 ||
+		    local_time->tm_mday < 1 || local_time->tm_year > 9999 ||
+		    local_time->tm_mon > 12 || local_time->tm_mday > 31)
+			errx(1, "illegal date format: use yyyy-mm-dd");
+
+		local_time->tm_year -= 1900;
+		local_time->tm_mon -= 1;
+		local_time->tm_hour = 0;
+		local_time->tm_min = 0;
+		local_time->tm_sec = 0;
+		local_time->tm_isdst = -1;
+		if (mktime(local_time) == -1 && local_time->tm_wday == -1)
+			errx(1, "specified date is outside allowed range");
+	}
+
 	if (month)
 		monthly(month, year);
 	else if (julian)
@@ -298,8 +333,13 @@ isoweek(int day, int month, int year)
 void
 monthly(int month, int year)
 {
-	int col, row, len, days[MAXDAYS], firstday;
-	char *p, lineout[30];
+	int col, row, len, hl_len, d, days[MAXDAYS], firstday;
+
+	/*
+	 * At least 9 bytes for standout mode escape sequence. What is a safe
+	 * size for this? Could this differ from terminal to terminal?
+	 */
+	char *p, lineout[48];
 
 	day_array(month, year, days);
 	(void)snprintf(lineout, sizeof(lineout), "%s %d",
@@ -310,15 +350,21 @@ monthly(int month, int year)
 	    lineout, day_headings);
 	for (row = 0; row < 6; row++) {
 		firstday = SPACE;
+		hl_len = 0;
 		for (col = 0, p = lineout; col < 7; col++,
 		    p += julian ? J_DAY_LEN : DAY_LEN) {
-			if (firstday == SPACE && days[row * 7 + col] != SPACE)
-				firstday = days[row * 7 + col];
-			ascii_day(p, days[row * 7 + col]);
+			d = days[row * 7 + col];
+			if (firstday == SPACE && d != SPACE)
+				firstday = d;
+			if (is_today(d, month, year) && highlight) {
+				highlight_day(p, d, &hl_len);
+				p += hl_len;
+			} else
+				ascii_day(p, d);
 		}
 		*p = '\0';
 		trim_trailing_spaces(lineout);
-		(void)printf("%-20s", lineout);
+		(void)printf("%-*s", 20 + hl_len, lineout);
 		if (wflag && firstday != SPACE)
 			printf(" [%2d]", week(firstday, month, year));
 		printf("\n");
@@ -328,8 +374,13 @@ monthly(int month, int year)
 void
 j_yearly(int year)
 {
-	int col, *dp, i, month, row, which_cal;
+	int col, *dp, i, month, row, which_cal, hl_len;
 	int days[12][MAXDAYS];
+
+	/*
+	 * At least 9 bytes for standout mode escape sequence. What is a safe
+	 * size for this? Could this differ from terminal to terminal?
+	 */
 	char *p, lineout[80];
 
 	(void)snprintf(lineout, sizeof(lineout), "%d", year);
@@ -346,15 +397,30 @@ j_yearly(int year)
 		    J_HEAD_SEP, "", day_headings);
 
 		for (row = 0; row < 6; row++) {
+			hl_len = 0;
 			for (which_cal = 0; which_cal < 2; which_cal++) {
-				p = lineout + which_cal * (J_WEEK_LEN + 2);
+				p = lineout + which_cal * (J_WEEK_LEN + 2) + hl_len;
 				dp = &days[month + which_cal][row * 7];
-				for (col = 0; col < 7; col++, p += J_DAY_LEN)
-					ascii_day(p, *dp++);
+				for (col = 0; col < 7; col++, p += J_DAY_LEN) {
+					if (is_today(*dp, month + which_cal + 1,
+					    year) && highlight) {
+						highlight_day(p, *dp++, &hl_len);
+						p += hl_len;
+					} else
+						ascii_day(p, *dp++);
+				}
 			}
 			*p = '\0';
 			trim_trailing_spaces(lineout);
 			(void)printf("%s\n", lineout);
+
+			/*
+			 * Highlighting a day causes escape sequences to
+			 * misalign the rows. Rather than tracking offsets to
+			 * overwrite rows day-by-day, clear the row.
+			 */
+			if (highlight)
+				(void)memset(lineout, ' ', sizeof(lineout) - 1);
 		}
 	}
 	(void)printf("\n");
@@ -363,9 +429,14 @@ j_yearly(int year)
 void
 yearly(int year)
 {
-	int col, *dp, i, month, row, which_cal, week_len, wn, firstday;
+	int col, *dp, i, month, row, which_cal, week_len, hl_len, wn, firstday;
 	int days[12][MAXDAYS];
-	char *p, lineout[81];
+
+	/*
+	 * At least 9 bytes for standout mode escape sequence. What is a safe
+	 * size for this? Could this differ from terminal to terminal?
+	 */
+	char *p, lineout[96];
 
 	week_len = WEEK_LEN;
 	if (wflag)
@@ -386,15 +457,21 @@ yearly(int year)
 		    HEAD_SEP + (wflag ? WEEKNUMBER_LEN : 0), "", day_headings);
 
 		for (row = 0; row < 6; row++) {
+			hl_len = 0;
 			for (which_cal = 0; which_cal < 3; which_cal++) {
-				p = lineout + which_cal * (week_len + 2);
+				p = lineout + which_cal * (week_len + 2) + hl_len;
 
 				dp = &days[month + which_cal][row * 7];
 				firstday = SPACE;
 				for (col = 0; col < 7; col++, p += DAY_LEN) {
 					if (firstday == SPACE && *dp != SPACE)
 						firstday = *dp;
-					ascii_day(p, *dp++);
+					if (is_today(*dp, month + which_cal + 1,
+					    year) && highlight) {
+						highlight_day(p, *dp++, &hl_len);
+						p += hl_len;
+					} else
+						ascii_day(p, *dp++);
 				}
 				if (wflag && firstday != SPACE) {
 					wn = week(firstday,
@@ -408,6 +485,14 @@ yearly(int year)
 			*p = '\0';
 			trim_trailing_spaces(lineout);
 			(void)printf("%s\n", lineout);
+
+			/*
+			 * Highlighting a day causes escape sequences to
+			 * misalign the rows. Rather than tracking offsets to
+			 * overwrite rows day-by-day, clear the row.
+			 */
+			if (highlight)
+				(void)memset(lineout, ' ', sizeof(lineout) - 1);
 		}
 	}
 	(void)printf("\n");
@@ -514,6 +599,50 @@ ascii_day(char *p, int day)
 }
 
 void
+highlight_day(char *p, int day, int *hl_len)
+{
+	const char *term_so, *term_se;
+	char cbuf[512];
+	char tbuf[1024], *b;
+
+	term_se = term_so = NULL;
+
+	/*
+	 * why NULL as name here?
+	 * termcap(3) doesn't seem document this
+	 */
+	if (tgetent(tbuf, NULL) == 1) {
+		b = cbuf;
+
+		/*
+		 * why not NULL here for area instead of &b?
+		 * cbuf is never used
+		 */
+		term_so = tgetstr("so", &b);
+		term_se = tgetstr("se", &b);
+	}
+
+	if (term_so != NULL && term_se != NULL) {
+		/* highlight on */
+		memcpy(p, term_so, strlen(term_so));
+		p += strlen(term_so);
+
+		/* the actual text */
+		ascii_day(p, day);
+		p += (julian ? J_DAY_LEN : DAY_LEN) - 1;
+
+		/* highlight off */
+		memcpy(p, term_se, strlen(term_se));
+		p += strlen(term_se);
+		*p = ' ';
+		*hl_len = strlen(term_so) + strlen(term_se);
+	} else {
+		ascii_day(p, day);
+		*hl_len = 0;
+	}
+}
+
+void
 trim_trailing_spaces(char *s)
 {
 	char *p;
@@ -539,8 +668,7 @@ center(const char *str, int len, int sep
 void
 usage(void)
 {
-
-	(void)fprintf(stderr, "usage: cal [-jmwy] [month] [year]\n");
+	(void)fprintf(stderr, "usage: cal [-hjmwy] [month] [year]\n");
 	exit(1);
 }
 
@@ -560,4 +688,12 @@ parsemonth(const char *s)
 	if (v <= 0 || v > 12)
 		errx(1, "invalid month: use 1-12 or a name");
 	return (v);
+}
+
+int
+is_today(int day, int month, int year)
+{
+	return (year == local_time->tm_year + 1900 &&
+	        month == local_time->tm_mon + 1 &&
+	        day == (julian ? local_time->tm_yday + 1 : local_time->tm_mday));
 }

• cal_test.sh (3K)