From 3e666656dc9ac50dd3bf36d35d70c403dd860c9f Mon Sep 17 00:00:00 2001
From: P. J. McDermott <pj@pehjota.net>
Date: Wed, 16 Mar 2022 20:06:49 -0400
Subject: main: Use difftime() instead of comparing time_t

C99 § 7.23.1 defines time_t as an "arithmetic [type] capable of
representing times" with "implementation-defined" "range and precision
of times representable".  It doesn't require that time1 > time0 for
time1 later than time0.

See also:
http://computer-programming-forum.com/47-c-language/8d7dec65bd1ddafe.htm
---
diff --git a/configure.ac b/configure.ac
index 3ff9f79..7c94fbd 100644
--- a/configure.ac
+++ b/configure.ac
@@ -38,8 +38,9 @@ test -d "${srcdir}/.git" || CFLAGS="${save_CFLAGS}"
 funcs_missing=false
 AC_CHECK_FUNCS(
 	[\
-		calloc fprintf fputs free isspace localtime mktime printf \
-		realloc setvbuf sleep strerror strlen strftime strptime time
+		calloc difftime fprintf fputs free isspace localtime mktime \
+		printf realloc setvbuf sleep strerror strlen strftime strptime \
+		time
 	],
 	[],
 	[funcs_missing=true])
diff --git a/src/main.c b/src/main.c
index 10630e8..4abf977 100644
--- a/src/main.c
+++ b/src/main.c
@@ -172,7 +172,7 @@ main(int argc, char * const argv[])
 	char   *buf;
 	time_t  arg;
 	time_t  now;
-	time_t  dif;
+	double  dif;
 
 	optind = 1;
 	opterr = 0;
@@ -218,14 +218,14 @@ main(int argc, char * const argv[])
 	free(buf);
 
 	time(&now);
-	dif = arg - now;
+	dif = difftime(arg, now);
 	if (dif >= 1000 * 24 * 60 * 60) {
 		fputs("Date too far in the future\n", stderr);
 		return EXIT_FAILURE;
 	}
 
 	setvbuf(stdout, NULL, _IONBF, 0);
-	while (arg > now) {
+	while (dif > 0) {
 		printf("\r%03d:%02d:%02d:%02d",
 				(int) dif / 60 / 60 / 24,
 				(int) dif / 60 / 60 % 24,
@@ -233,7 +233,7 @@ main(int argc, char * const argv[])
 				(int) dif % 60);
 		sleep(1);
 		time(&now);
-		dif = arg - now;
+		dif = difftime(arg, now);
 	}
 	printf("\r000:00:00:00\n");
 
--
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