From 4a3bbead361b7c58c8259c3fcfc13a04866fd622 Mon Sep 17 00:00:00 2001
From: P. J. McDermott <pj@pehjota.net>
Date: Mon, 28 Mar 2022 19:49:46 -0400
Subject: datetime: Don't use mktime()/localtime() shortcuts

---
(limited to 'src')

diff --git a/src/datetime.c b/src/datetime.c
index ccf4c29..2159bb2 100644
--- a/src/datetime.c
+++ b/src/datetime.c
@@ -86,7 +86,13 @@ _datetime_reset_tm(struct tm *tm)
 static void
 _datetime_normalize(struct tm *now_tm, struct tm *arg_tm, time_t *arg_sec)
 {
-	int wday;
+	int days_in_mon[12] = {31, 28, 31, 30, 31, 30,
+	                       31, 31, 30, 31, 30, 31};
+
+	if (now_tm->tm_year % 4 == 0 && (now_tm->tm_year % 100 != 0 ||
+				now_tm->tm_year % 400 == 0)) {
+		days_in_mon[1] = 29;
+	}
 
 	if (arg_tm->tm_sec == INT_MIN) {
 		arg_tm->tm_sec = 0;
@@ -100,29 +106,37 @@ _datetime_normalize(struct tm *now_tm, struct tm *arg_tm, time_t *arg_sec)
 		*arg_sec = mktime(arg_tm);
 		if (*arg_sec <= mktime(now_tm)) {
 			/* Specified time already happened today; use tomorrow.
-			 * Adding the number of seconds in a day is a shortcut
-			 * that ignores leap seconds.  One better method would
-			 * be to increment tm_mday % days in tm_mon, etc. */
-			*arg_sec += 60 * 60 * 24;
-			now_tm = localtime(arg_sec);
-			arg_tm->tm_year = now_tm->tm_year;
-			arg_tm->tm_mon  = now_tm->tm_mon;
-			arg_tm->tm_mday = now_tm->tm_mday;
+			 */
+			++arg_tm->tm_mday;
+			arg_tm->tm_wday = (arg_tm->tm_wday + 1) % 7;
+			if (arg_tm->tm_mday > days_in_mon[arg_tm->tm_mon]) {
+				arg_tm->tm_mday = 1;
+				++arg_tm->tm_mon;
+				if (arg_tm->tm_mon >= 12) {
+					arg_tm->tm_mon = 0;
+					++arg_tm->tm_year;
+				}
+			}
 			*arg_sec = mktime(arg_tm);
 		}
 	} else if (arg_tm->tm_mday == INT_MIN) {
-		/* Only a weekday specified; try tomorrow or next week.  Uses
-		 * same shortcut as above. */
-		wday = arg_tm->tm_wday;
+		/* Only a weekday specified; try tomorrow or next week. */
 		arg_tm->tm_year = now_tm->tm_year;
 		arg_tm->tm_mon  = now_tm->tm_mon;
 		arg_tm->tm_mday = now_tm->tm_mday;
-		arg_tm->tm_wday = now_tm->tm_wday;
-		if (wday <= now_tm->tm_wday) {
-			wday += 7;
+		if (arg_tm->tm_wday <= now_tm->tm_wday) {
+			arg_tm->tm_mday += 7;
+		}
+		arg_tm->tm_mday += arg_tm->tm_wday - now_tm->tm_wday;
+		if (arg_tm->tm_mday > days_in_mon[arg_tm->tm_mon]) {
+			arg_tm->tm_mday -= days_in_mon[arg_tm->tm_mon];
+			++arg_tm->tm_mon;
+			if (arg_tm->tm_mon >= 12) {
+				arg_tm->tm_mon = 0;
+				++arg_tm->tm_year;
+			}
 		}
 		*arg_sec = mktime(arg_tm);
-		*arg_sec += (60 * 60 * 24) * (wday - now_tm->tm_wday);
 	} else if (arg_tm->tm_year == INT_MIN) {
 		/* No year specified; try this year. */
 		arg_tm->tm_year = now_tm->tm_year;
--
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