diff --git a/phc2sys.8 b/phc2sys.8
index ad1bfa2..6933900 100644
--- a/phc2sys.8
+++ b/phc2sys.8
@@ -24,6 +24,8 @@ phc2sys \- synchronize two clocks
 .BI \-R " update-rate"
 ] [
 .BI \-N " clock-readings"
+] [
+.BI \-O " offset"
 ]
 
 .SH DESCRIPTION
@@ -84,6 +86,10 @@ the fastest reading is used to update the slave clock, this is useful to
 minimize the error caused by random delays in scheduling and bus utilization.
 The default is 5.
 .TP
+.BI \-O " offset"
+Specify the offset between the slave and master times in seconds.
+The default is 0 seconds.
+.TP
 .BI \-h
 Display a help message.
 
diff --git a/phc2sys.c b/phc2sys.c
index d1df723..fced164 100644
--- a/phc2sys.c
+++ b/phc2sys.c
@@ -241,6 +241,7 @@ static void usage(char *progname)
 		" -I [ki]        integration constant (0.3)\n"
 		" -R [rate]      slave clock update rate in HZ (1)\n"
 		" -N [num]       number of master clock readings per update (5)\n"
+		" -O [offset]    slave-master time offset (0)\n"
 		" -h             prints this message and exits\n"
 		"\n",
 		progname);
@@ -253,12 +254,12 @@ int main(int argc, char *argv[])
 	clockid_t src = CLOCK_INVALID, dst = CLOCK_REALTIME;
 	uint64_t phc_ts;
 	int64_t phc_offset;
-	int c, phc_readings = 5, phc_rate = 1;
+	int c, phc_readings = 5, phc_rate = 1, sync_offset = 0;
 
 	/* Process the command line arguments. */
 	progname = strrchr(argv[0], '/');
 	progname = progname ? 1+progname : argv[0];
-	while (EOF != (c = getopt(argc, argv, "c:d:hs:P:I:R:N:i:"))) {
+	while (EOF != (c = getopt(argc, argv, "c:d:hs:P:I:R:N:O:i:"))) {
 		switch (c) {
 		case 'c':
 			dst = clock_open(optarg);
@@ -281,6 +282,9 @@ int main(int argc, char *argv[])
 		case 'N':
 			phc_readings = atoi(optarg);
 			break;
+		case 'O':
+			sync_offset = atoi(optarg);
+			break;
 		case 'i':
 			ethdev = optarg;
 			break;
@@ -311,6 +315,7 @@ int main(int argc, char *argv[])
 		struct timespec now;
 		if (clock_gettime(src, &now))
 			perror("clock_gettime");
+		now.tv_sec += sync_offset;
 		if (clock_settime(dst, &now))
 			perror("clock_settime");
 	}
@@ -323,6 +328,7 @@ int main(int argc, char *argv[])
 		if (!read_phc(src, dst, phc_readings, &phc_offset, &phc_ts)) {
 			continue;
 		}
+		phc_offset -= sync_offset * NS_PER_SEC;
 		do_servo(&servo, dst, phc_offset, phc_ts, kp, ki);
 		show_servo(stdout, "phc", phc_offset, phc_ts);
 	}