diff --git a/phc2sys.8 b/phc2sys.8
index 43e3abc..92081cf 100644
--- a/phc2sys.8
+++ b/phc2sys.8
@@ -109,10 +109,10 @@ 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 set automatically with the
-.B \-w
-option, 0 otherwise.
+Specify the offset between the slave and master times in seconds.  See
+.SM
+.B TIME SCALE USAGE
+below.
 .TP
 .BI \-u " summary-updates"
 Specify the number of clock updates included in summary statistics. The
diff --git a/phc2sys.c b/phc2sys.c
index 1dbe2bc..309ee6c 100644
--- a/phc2sys.c
+++ b/phc2sys.c
@@ -684,6 +684,13 @@ int main(int argc, char *argv[])
 		return -1;
 	}
 
+	if (!wait_sync && !forced_sync_offset) {
+		fprintf(stderr,
+			"time offset must be specified using -w or -O\n");
+		usage(progname);
+		return -1;
+	}
+
 	if (dst_clock.stats_max_count > 0) {
 		dst_clock.offset_stats = stats_create();
 		dst_clock.freq_stats = stats_create();