diff --git a/phc2sys.8 b/phc2sys.8
index 26b37a8..ad1bfa2 100644
--- a/phc2sys.8
+++ b/phc2sys.8
@@ -53,8 +53,8 @@ is started or the
 option should be used too. 
 .TP
 .BI \-s " phc-device"
-Specify the device of the master clock (e.g. /dev/ptp0). When this option is
-used together with the
+Specify the master clock by device (e.g. /dev/ptp0) or name (e.g. CLOCK_REALTIME
+for the system clock). When this option is used together with the
 .B \-d
 option, the master clock is read only on start to fix an offset over 0.5
 seconds which cannot be fixed with PPS alone.
@@ -65,8 +65,8 @@ Similar to the
 option, but specified by the interface which provides the master clock. 
 .TP
 .BI \-c " phc-device"
-Specify the device of the slave clock (e.g. /dev/ptp1). The default slave clock
-is the system clock (CLOCK_REALTIME).
+Specify the slave clock by device (e.g. /dev/ptp1) or name. The default is
+CLOCK_REALTIME (the system clock).
 .TP
 .BI \-P " kp"
 Specify the proportional constant of the PI controller. The default is 0.7.
diff --git a/phc2sys.c b/phc2sys.c
index 26a0c2b..7bafbed 100644
--- a/phc2sys.c
+++ b/phc2sys.c
@@ -44,7 +44,17 @@
 
 static clockid_t clock_open(char *device)
 {
-	int fd = open(device, O_RDWR);
+	int fd;
+
+	if (device[0] != '/') {
+		if (!strcasecmp(device, "CLOCK_REALTIME"))
+			return CLOCK_REALTIME;
+
+		fprintf(stderr, "unknown clock %s\n", device);
+		return CLOCK_INVALID;
+	}
+
+	fd = open(device, O_RDWR);
 	if (fd < 0) {
 		fprintf(stderr, "cannot open %s: %m\n", device);
 		return CLOCK_INVALID;