diff --git a/misc.c b/misc.c
index a8e87430..f2135803 100644
--- a/misc.c
+++ b/misc.c
@@ -2399,15 +2399,26 @@ parse_absolute_time(const char *s, uint64_t *tp)
 	struct tm tm;
 	time_t tt;
 	char buf[32], *fmt;
+	const char *cp;
+	size_t l;
+	int is_utc = 0;
 
 	*tp = 0;
 
+	l = strlen(s);
+	if (l > 1 && strcasecmp(s + l - 1, "Z") == 0) {
+		is_utc = 1;
+		l--;
+	} else if (l > 3 && strcasecmp(s + l - 3, "UTC") == 0) {
+		is_utc = 1;
+		l -= 3;
+	}
 	/*
 	 * POSIX strptime says "The application shall ensure that there
 	 * is white-space or other non-alphanumeric characters between
 	 * any two conversion specifications" so arrange things this way.
 	 */
-	switch (strlen(s)) {
+	switch (l) {
 	case 8: /* YYYYMMDD */
 		fmt = "%Y-%m-%d";
 		snprintf(buf, sizeof(buf), "%.4s-%.2s-%.2s", s, s + 4, s + 6);
@@ -2427,10 +2438,15 @@ parse_absolute_time(const char *s, uint64_t *tp)
 	}
 
 	memset(&tm, 0, sizeof(tm));
-	if (strptime(buf, fmt, &tm) == NULL)
-		return SSH_ERR_INVALID_FORMAT;
-	if ((tt = mktime(&tm)) < 0)
+	if ((cp = strptime(buf, fmt, &tm)) == NULL || *cp != '\0')
 		return SSH_ERR_INVALID_FORMAT;
+	if (is_utc) {
+		if ((tt = timegm(&tm)) < 0)
+			return SSH_ERR_INVALID_FORMAT;
+	} else {
+		if ((tt = mktime(&tm)) < 0)
+			return SSH_ERR_INVALID_FORMAT;
+	}
 	/* success */
 	*tp = (uint64_t)tt;
 	return 0;
diff --git a/regress/unittests/misc/test_convtime.c b/regress/unittests/misc/test_convtime.c
index ef6fd77d..4794dbd9 100644
--- a/regress/unittests/misc/test_convtime.c
+++ b/regress/unittests/misc/test_convtime.c
@@ -20,6 +20,7 @@
 
 #include "log.h"
 #include "misc.h"
+#include "ssherr.h"
 
 void test_convtime(void);
 
@@ -27,6 +28,7 @@ void
 test_convtime(void)
 {
 	char buf[1024];
+	uint64_t t;
 
 	TEST_START("misc_convtime");
 	ASSERT_INT_EQ(convtime("0"), 0);
@@ -56,4 +58,64 @@ test_convtime(void)
 	ASSERT_INT_EQ(convtime("3550w5d3h14m8s"), -1);
 #endif
 	TEST_DONE();
+
+	/* XXX timezones/DST make verification of this tricky */
+	/* XXX maybe setenv TZ and tzset() to make it unambiguous? */
+	TEST_START("misc_parse_absolute_time");
+	ASSERT_INT_EQ(parse_absolute_time("20000101", &t), 0);
+	ASSERT_INT_EQ(parse_absolute_time("200001011223", &t), 0);
+	ASSERT_INT_EQ(parse_absolute_time("20000101122345", &t), 0);
+
+	/* forced UTC TZ */
+	ASSERT_INT_EQ(parse_absolute_time("20000101Z", &t), 0);
+	ASSERT_U64_EQ(t, 946684800);
+	ASSERT_INT_EQ(parse_absolute_time("200001011223Z", &t), 0);
+	ASSERT_U64_EQ(t, 946729380);
+	ASSERT_INT_EQ(parse_absolute_time("20000101122345Z", &t), 0);
+	ASSERT_U64_EQ(t, 946729425);
+	ASSERT_INT_EQ(parse_absolute_time("20000101UTC", &t), 0);
+	ASSERT_U64_EQ(t, 946684800);
+	ASSERT_INT_EQ(parse_absolute_time("200001011223UTC", &t), 0);
+	ASSERT_U64_EQ(t, 946729380);
+	ASSERT_INT_EQ(parse_absolute_time("20000101122345UTC", &t), 0);
+	ASSERT_U64_EQ(t, 946729425);
+
+	/* Bad month */
+	ASSERT_INT_EQ(parse_absolute_time("20001301", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("20000001", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	/* Incomplete */
+	ASSERT_INT_EQ(parse_absolute_time("2", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("2000", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("20000", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("200001", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("2000010", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("200001010", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	/* Bad day, hour, minute, second */
+	ASSERT_INT_EQ(parse_absolute_time("20000199", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("200001019900", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("200001010099", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("20000101000099", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	/* Invalid TZ specifier */
+	ASSERT_INT_EQ(parse_absolute_time("20000101ZZ", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("20000101PDT", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("20000101U", &t),
+	    SSH_ERR_INVALID_FORMAT);
+	ASSERT_INT_EQ(parse_absolute_time("20000101UTCUTC", &t),
+	    SSH_ERR_INVALID_FORMAT);
+
+	TEST_DONE();
 }
diff --git a/ssh-keygen.1 b/ssh-keygen.1
index 5f429813..6aeab1cb 100644
--- a/ssh-keygen.1
+++ b/ssh-keygen.1
@@ -511,8 +511,11 @@ Print the full public key to standard output after signature verification.
 .It Cm verify-time Ns = Ns Ar timestamp
 Specifies a time to use when validating signatures instead of the current
 time.
-The time may be specified as a date in YYYYMMDD format or a time
-in YYYYMMDDHHMM[SS] format.
+The time may be specified as a date or time in the YYYYMMDD[Z] or
+in YYYYMMDDHHMM[SS][Z] formats.
+Dates and times will be interpreted in the current system time zone unless
+suffixed with a Z character, which causes them to be interpreted in the
+UTC time zone.
 .El
 .Pp
 The
@@ -603,31 +606,67 @@ A validity interval may consist of a single time, indicating that the
 certificate is valid beginning now and expiring at that time, or may consist
 of two times separated by a colon to indicate an explicit time interval.
 .Pp
-The start time may be specified as the string
+The start time may be specified as:
+.Bl -bullet -compact
+.It
+The string
 .Dq always
-to indicate the certificate has no specified start time,
-a date in YYYYMMDD format, a time in YYYYMMDDHHMM[SS] format,
-a relative time (to the current time) consisting of a minus sign followed by
-an interval in the format described in the
+to indicate the certificate has no specified start time.
+.It
+A date or time in the system time zone formatted as YYYYMMDD or
+YYYYMMDDHHMM[SS].
+.It
+A date or time in the UTC time zone as YYYYMMDDZ or YYYYMMDDHHMM[SS]Z.
+.It
+A relative time before the current system time consisting of a minus sign
+followed by an interval in the format described in the
 TIME FORMATS section of
 .Xr sshd_config 5 .
+.It
+A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as a hexadecimal
+number beginning with
+.Dq 0x .
+.El
 .Pp
-The end time may be specified as a YYYYMMDD date, a YYYYMMDDHHMM[SS] time,
-a relative time starting with a plus character or the string
+The end time may be specified similarly to the start time:
+.Bl -bullet -compact
+.It
+The string
 .Dq forever
-to indicate that the certificate has no expiry date.
+to indicate the certificate has no specified end time.
+.It
+A date or time in the system time zone formatted as YYYYMMDD or
+YYYYMMDDHHMM[SS].
+.It
+A date or time in the UTC time zone as YYYYMMDDZ or YYYYMMDDHHMM[SS]Z.
+.It
+A relative time after the current system time consisting of a plus sign
+followed by an interval in the format described in the
+TIME FORMATS section of
+.Xr sshd_config 5 .
+.It
+A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as a hexadecimal
+number beginning with
+.Dq 0x .
+.El
 .Pp
 For example:
-.Dq +52w1d
-(valid from now to 52 weeks and one day from now),
-.Dq -4w:+4w
-(valid from four weeks ago to four weeks from now),
-.Dq 20100101123000:20110101123000
-(valid from 12:30 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011),
-.Dq -1d:20110101
-(valid from yesterday to midnight, January 1st, 2011),
-.Dq -1m:forever
-(valid from one minute ago and never expiring).
+.Bl -tag -width Ds
+.It +52w1d
+Valid from now to 52 weeks and one day from now.
+.It -4w:+4w
+Valid from four weeks ago to four weeks from now.
+.It 20100101123000:20110101123000
+Valid from 12:30 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011.
+.It 20100101123000Z:20110101123000Z
+Similar, but interpreted in the UTC time zone rather than the system time zone.
+.It -1d:20110101
+Valid from yesterday to midnight, January 1st, 2011.
+.It 0x1:0x2000000000
+Valid from roughly early 1970 to May 2033.
+.It -1m:forever
+Valid from one minute ago and never expiring.
+.El
 .It Fl v
 Verbose mode.
 Causes
@@ -1206,7 +1245,10 @@ signature object and presented on the verification command-line must
 match the specified list before the key will be considered acceptable.
 .It Cm valid-after Ns = Ns "timestamp"
 Indicates that the key is valid for use at or after the specified timestamp,
-which may be a date in YYYYMMDD format or a time in YYYYMMDDHHMM[SS] format.
+which may be a date or time in the YYYYMMDD[Z] or YYYYMMDDHHMM[SS][Z] formats.
+Dates and times will be interpreted in the current system time zone unless
+suffixed with a Z character, which causes them to be interpreted in the UTC
+time zone.
 .It Cm valid-before Ns = Ns "timestamp"
 Indicates that the key is valid for use at or before the specified timestamp.
 .El
diff --git a/ssh-keygen.c b/ssh-keygen.c
index 20b321cc..9b2beda0 100644
--- a/ssh-keygen.c
+++ b/ssh-keygen.c
@@ -1916,6 +1916,21 @@ parse_relative_time(const char *s, time_t now)
 	return now + (u_int64_t)(secs * mul);
 }
 
+static void
+parse_hex_u64(const char *s, uint64_t *up)
+{
+	char *ep;
+	unsigned long long ull;
+
+	errno = 0;
+	ull = strtoull(s, &ep, 16);
+	if (*s == '\0' || *ep != '\0')
+		fatal("Invalid certificate time: not a number");
+	if (errno == ERANGE && ull == ULONG_MAX)
+		fatal_fr(SSH_ERR_SYSTEM_ERROR, "Invalid certificate time");
+	*up = (uint64_t)ull;
+}
+
 static void
 parse_cert_times(char *timespec)
 {
@@ -1938,8 +1953,8 @@ parse_cert_times(char *timespec)
 
 	/*
 	 * from:to, where
-	 * from := [+-]timespec | YYYYMMDD | YYYYMMDDHHMMSS | "always"
-	 *   to := [+-]timespec | YYYYMMDD | YYYYMMDDHHMMSS | "forever"
+	 * from := [+-]timespec | YYYYMMDD | YYYYMMDDHHMMSS | 0x... | "always"
+	 *   to := [+-]timespec | YYYYMMDD | YYYYMMDDHHMMSS | 0x... | "forever"
 	 */
 	from = xstrdup(timespec);
 	to = strchr(from, ':');
@@ -1951,6 +1966,8 @@ parse_cert_times(char *timespec)
 		cert_valid_from = parse_relative_time(from, now);
 	else if (strcmp(from, "always") == 0)
 		cert_valid_from = 0;
+	else if (strncmp(from, "0x", 2) == 0)
+		parse_hex_u64(from, &cert_valid_from);
 	else if (parse_absolute_time(from, &cert_valid_from) != 0)
 		fatal("Invalid from time \"%s\"", from);
 
@@ -1958,6 +1975,8 @@ parse_cert_times(char *timespec)
 		cert_valid_to = parse_relative_time(to, now);
 	else if (strcmp(to, "forever") == 0)
 		cert_valid_to = ~(u_int64_t)0;
+	else if (strncmp(to, "0x", 2) == 0)
+		parse_hex_u64(to, &cert_valid_to);
 	else if (parse_absolute_time(to, &cert_valid_to) != 0)
 		fatal("Invalid to time \"%s\"", to);
 
diff --git a/sshd.8 b/sshd.8
index 2b50514e..8ccc5bc0 100644
--- a/sshd.8
+++ b/sshd.8
@@ -533,8 +533,9 @@ controlled via the
 option.
 .It Cm expiry-time="timespec"
 Specifies a time after which the key will not be accepted.
-The time may be specified as a YYYYMMDD date or a YYYYMMDDHHMM[SS] time
-in the system time-zone.
+The time may be specified as a YYYYMMDD[Z] date or a YYYYMMDDHHMM[SS][Z] time.
+Dates and times will be interpreted in the system time zone unless suffixed
+by a Z character, in which case they will be interpreted in the UTC time zone.
 .It Cm from="pattern-list"
 Specifies that in addition to public key authentication, either the canonical
 name of the remote host or its IP address must be present in the