diff --git a/notecard/main.go b/notecard/main.go
index ce66293..a694eaa 100644
--- a/notecard/main.go
+++ b/notecard/main.go
@@ -93,6 +93,7 @@ func getFlagGroups() []lib.FlagGroup {
 				lib.GetFlagByName("playtime"),
 				lib.GetFlagByName("commtest"),
 				lib.GetFlagByName("echo"),
+				lib.GetFlagByName("rtc"),
 				lib.GetFlagByName("binpack"),
 				lib.GetFlagByName("pcap"),
 			},
@@ -240,6 +241,8 @@ func main() {
 	flag.StringVar(&actionSideload, "sideload", "", "side-load a .bin or .bins into the Notecard's storage")
 	var actionEcho int
 	flag.IntVar(&actionEcho, "echo", 0, "perform <N> iterations of a communications reliability test to the Notecard")
+	var actionRTC int
+	flag.IntVar(&actionRTC, "rtc", 0, "measure the Notecard's RTC drift against the host clock, once per second for <N> seconds")
 	var actionVersion bool
 	flag.BoolVar(&actionVersion, "version", false, "print the current version of the CLI")
 	var actionPcap string
@@ -919,6 +922,10 @@ func main() {
 		err = echo(actionEcho)
 	}
 
+	if err == nil && actionRTC != 0 {
+		err = rtc(actionRTC, actionVerbose)
+	}
+
 	if err == nil && actionVersion {
 		fmt.Printf("Notecard CLI Version: %s\n", version)
 	}
diff --git a/notecard/rtc.go b/notecard/rtc.go
new file mode 100644
index 0000000..2ba9649
--- /dev/null
+++ b/notecard/rtc.go
@@ -0,0 +1,114 @@
+// Copyright 2017 Blues Inc.  All rights reserved.
+// Use of this source code is governed by licenses granted by the
+// copyright holder including that found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/blues/note-go/notecard"
+)
+
+// rtcProbes is the number of rapid card.time reads taken per measurement. We keep only the one
+// with the smallest host round-trip time, because that sample has the tightest alignment between
+// the Notecard's reported clock and the host clock. This is the standard NTP-style minimum
+// round-trip filter, and it removes most of the per-transaction USB/serial latency jitter.
+const rtcProbes = 8
+
+// rtcSample reads the Notecard's high-resolution clock using {"req":"card.time","now":true}, which
+// returns the time in the "value" field as <epoch-seconds>.<six-digits-of-microseconds>. It probes
+// several times in quick succession and returns the reading with the lowest round-trip latency,
+// expressed in microseconds. cardUs is the Notecard's clock; hostUs is the host clock at the
+// midpoint of that round trip, which is our best estimate of the instant the Notecard sampled.
+func rtcSample() (cardUs int64, hostUs int64, err error) {
+	bestRTTUs := int64(-1)
+	for p := 0; p < rtcProbes; p++ {
+		before := time.Now()
+		var rsp notecard.Request
+		rsp, err = card.TransactionRequest(notecard.Request{Req: "card.time", Now: true})
+		after := time.Now()
+		if err != nil {
+			return
+		}
+		rttUs := after.Sub(before).Microseconds()
+		if bestRTTUs < 0 || rttUs < bestRTTUs {
+			bestRTTUs = rttUs
+			cardUs = int64(rsp.Value * 1000000)
+			hostUs = before.Add(after.Sub(before) / 2).UnixMicro()
+		}
+	}
+	return
+}
+
+// rtc measures the drift of the Notecard's real-time clock relative to the host computer's clock,
+// emitting one line per second for the specified number of seconds. This assumes the host has an
+// accurate, high-resolution clock to measure against.
+func rtc(seconds int, verbose bool) (err error) {
+
+	// Quiet the debug output unless the user asked for verbosity, so that our once-per-second
+	// lines aren't interleaved with transaction tracing.
+	if !verbose {
+		card.DebugOutput(false, false)
+	}
+
+	// Establish the origin against which all subsequent samples are measured: the host clock and
+	// the Notecard's clock at the same instant, both in microseconds. We measure each sample's
+	// elapsed time relative to this origin so the regression below works with small numbers.
+	originCardUs, originHostUs, err := rtcSample()
+	if err != nil {
+		return
+	}
+
+	// Rather than deriving the drift from just the origin and the current sample (which exposes
+	// the full per-sample measurement noise of those two readings), we fit a least-squares line
+	// through every sample collected so far. We regress the drift itself (microseconds of drift
+	// since start) against the host-elapsed time; the slope of that line is the drift rate. Using
+	// all N samples averages out the per-sample jitter, so the reported figures stabilize as the
+	// run lengthens. We accumulate the regression sums incrementally, seeding them with the origin
+	// point (host-elapsed 0, drift 0).
+	var n, sumX, sumD, sumXX, sumXD float64
+	n = 1 // the origin point (x=0, drift=0) contributes nothing to the sums but counts toward n
+
+	// Emit one measurement per second, on a one-second cadence.
+	ticker := time.NewTicker(time.Second)
+	defer ticker.Stop()
+	for i := 1; i <= seconds; i++ {
+		<-ticker.C
+
+		var cardUs, hostUs int64
+		cardUs, hostUs, err = rtcSample()
+		if err != nil {
+			return
+		}
+
+		// Microseconds elapsed on the host, and microseconds of drift, both since the origin.
+		// Drift is how much further the Notecard's clock has advanced than the host's clock; a
+		// positive value means the Notecard's RTC is running fast.
+		x := float64(hostUs - originHostUs)
+		drift := float64((cardUs - originCardUs) - (hostUs - originHostUs))
+
+		// Accumulate this sample into the running least-squares fit of drift against host-elapsed.
+		n++
+		sumX += x
+		sumD += drift
+		sumXX += x * x
+		sumXD += x * drift
+
+		// Least-squares slope of drift vs host-elapsed: microseconds of drift accrued per
+		// microsecond of host time. Scale that rate to the requested units.
+		avgDriftMsPerSecond := 0.0
+		driftSecsPerDay := 0.0
+		denom := n*sumXX - sumX*sumX
+		if denom != 0 {
+			driftRate := (n*sumXD - sumX*sumD) / denom
+			avgDriftMsPerSecond = driftRate * 1000
+			driftSecsPerDay = driftRate * 86400
+		}
+
+		fmt.Printf("%d rtc: avgDriftMsPerSecond:%.3f driftSecsPerDay:%.3f\n", i, avgDriftMsPerSecond, driftSecsPerDay)
+	}
+
+	return
+}