diff --git a/lib/libutil/Makefile b/lib/libutil/Makefile
--- a/lib/libutil/Makefile
+++ b/lib/libutil/Makefile
@@ -18,7 +18,7 @@
 	login_class.c login_crypt.c login_ok.c login_times.c login_tty.c \
 	mntopts.c \
 	pidfile.c property.c pty.c pw_scan.c pw_util.c quotafile.c \
-	realhostname.c stub.c trimdomain.c uucplock.c
+	realhostname.c source_date_epoch.c stub.c trimdomain.c uucplock.c
 INCS=	libutil.h login_cap.h mntopts.h
 
 CFLAGS+= -DNO__SCCSID
diff --git a/lib/libutil/libutil.h b/lib/libutil/libutil.h
--- a/lib/libutil/libutil.h
+++ b/lib/libutil/libutil.h
@@ -63,6 +63,11 @@
 #define	_SIZE_T_DECLARED
 #endif
 
+#ifndef _TIME_T_DECLARED
+typedef	__time_t	time_t;
+#define	_TIME_T_DECLARED
+#endif
+
 #ifndef _UID_T_DECLARED
 typedef	__uid_t		uid_t;
 #define	_UID_T_DECLARED
@@ -133,6 +138,7 @@
 int	realhostname_sa(char *_host, size_t _hsize, struct sockaddr *_addr,
 	    int _addrlen);
 int	_secure_path(const char *_path, uid_t _uid, gid_t _gid);
+int	source_date_epoch(time_t *tp);
 void	trimdomain(char *_fullhost, int _hostsize);
 const char *
 	uu_lockerr(int _uu_lockresult);
diff --git a/lib/libutil/source_date_epoch.c b/lib/libutil/source_date_epoch.c
new file mode 100644
--- /dev/null
+++ b/lib/libutil/source_date_epoch.c
@@ -0,0 +1,49 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2025 The FreeBSD Foundation
+ *
+ * This software was developed by Klara, Inc. under sponsorship from the FreeBSD
+ * Foundation and the Sovereign Tech Agency.
+ */
+
+#include <sys/types.h>
+
+#include <errno.h>
+#include <inttypes.h>
+#include <libutil.h>
+#include <limits.h>
+#include <stdlib.h>
+
+int
+source_date_epoch(time_t *tp)
+{
+	intmax_t val;
+	char *end, *env;
+
+	env = getenv("SOURCE_DATE_EPOCH");
+	if (env == NULL)
+		return (1);
+
+	errno = 0;
+	val = strtoimax(env, &end, 10);
+	if (errno != 0)
+		return (-1);
+	if (end == env || *end != '\0') {
+		errno = EINVAL;
+		return (-1);
+	}
+
+	/* Check for truncation.  Unfortunately there is no TIME_T_MAX. */
+	_Static_assert(sizeof(time_t) == sizeof(int32_t) ||
+	    sizeof(time_t) == sizeof(int64_t),
+	    "time_t must be 32 or 64 bits");
+	if (val < 0 || (sizeof(time_t) == sizeof(int32_t) && val > INT32_MAX) ||
+	    (sizeof(time_t) == sizeof(int64_t) && val > INT64_MAX)) {
+		errno = ERANGE;
+		return (-1);
+	}
+
+	*tp = (time_t)val;
+	return (0);
+}