diff --git a/go/src/types/mlrval_functions_time.go b/go/src/types/mlrval_functions_time.go
index 9f016ae2c..71e5ea3c0 100644
--- a/go/src/types/mlrval_functions_time.go
+++ b/go/src/types/mlrval_functions_time.go
@@ -1,6 +1,8 @@
 package types
 
 import (
+	"fmt"
+	"regexp"
 	"time"
 
 	"github.com/lestrrat-go/strftime"
@@ -63,9 +65,12 @@ func MlrvalSec2GMTDate(input1 *Mlrval) *Mlrval {
 	return MlrvalStrftime(input1, MlrvalPointerFromString("%Y-%m-%d"))
 }
 
-// ----------------------------------------------------------------
+// ================================================================
 // Argument 1 is int/float seconds since the epoch.
 // Argument 2 is format string like "%Y-%m-%d %H:%M:%S".
+
+var extensionRegex = regexp.MustCompile("([1-9])S")
+
 func MlrvalStrftime(input1, input2 *Mlrval) *Mlrval {
 	epochSeconds, ok := input1.GetNumericToFloatValue()
 	if !ok {
@@ -74,18 +79,90 @@ func MlrvalStrftime(input1, input2 *Mlrval) *Mlrval {
 	if input2.mvtype != MT_STRING {
 		return MLRVAL_ERROR
 	}
-	formatString := input2.printrep
 
+	// Convert argument1 from float seconds since the epoch to a Go time.
 	inputTime := lib.EpochSecondsToTime(epochSeconds)
 
-	outputString, err := strftime.Format(formatString, inputTime)
+	// Convert argument 2 to a strftime format string.
+	//
+	// Miller fractional-second formats are like "%6S", and were so in the C
+	// implementation. However, in the strftime package we're using in the Go
+	// port, extension-formats are only a single byte so we need to rewrite
+	// them to "%6".
+	formatString := extensionRegex.ReplaceAllString(input2.printrep, "$1")
+
+	formatter, err := strftime.New(formatString, strftimeExtensions)
 	if err != nil {
 		return MLRVAL_ERROR
 	}
 
+	outputString := formatter.FormatString(inputTime)
+
 	return MlrvalPointerFromString(outputString)
 }
 
+// ----------------------------------------------------------------
+// This is support for %1S .. %9S in format strings, using github.com/lestrrat-go/strftime.
+
+var strftimeExtensions strftime.Option
+
+// This is a helper function for the appenders below, which let people get
+// 1..9 decimal places in the seconds of their strftime format strings.
+func specificationHelper(b []byte, t time.Time, sprintfFormat string, quotient int) []byte {
+	seconds := int(t.Second())
+	fractional := int(t.Nanosecond() / quotient)
+	secondsString := fmt.Sprintf("%02d", seconds)
+	b = append(b, secondsString...)
+	b = append(b, '.')
+	fractionalString := fmt.Sprintf(sprintfFormat, fractional)
+	b = append(b, fractionalString...)
+	return b
+}
+
+func init() {
+	appender1 := strftime.AppendFunc(func(b []byte, t time.Time) []byte {
+		return specificationHelper(b, t, "%01d", 100000000)
+	})
+	appender2 := strftime.AppendFunc(func(b []byte, t time.Time) []byte {
+		return specificationHelper(b, t, "%02d", 10000000)
+	})
+	appender3 := strftime.AppendFunc(func(b []byte, t time.Time) []byte {
+		return specificationHelper(b, t, "%03d", 1000000)
+	})
+	appender4 := strftime.AppendFunc(func(b []byte, t time.Time) []byte {
+		return specificationHelper(b, t, "%04d", 100000)
+	})
+	appender5 := strftime.AppendFunc(func(b []byte, t time.Time) []byte {
+		return specificationHelper(b, t, "%05d", 10000)
+	})
+	appender6 := strftime.AppendFunc(func(b []byte, t time.Time) []byte {
+		return specificationHelper(b, t, "%06d", 1000)
+	})
+	appender7 := strftime.AppendFunc(func(b []byte, t time.Time) []byte {
+		return specificationHelper(b, t, "%07d", 100)
+	})
+	appender8 := strftime.AppendFunc(func(b []byte, t time.Time) []byte {
+		return specificationHelper(b, t, "%09d", 10)
+	})
+	appender9 := strftime.AppendFunc(func(b []byte, t time.Time) []byte {
+		return specificationHelper(b, t, "%09d", 1)
+	})
+
+	ss := strftime.NewSpecificationSet()
+	ss.Set('1', appender1)
+	ss.Set('2', appender2)
+	ss.Set('3', appender3)
+	ss.Set('4', appender4)
+	ss.Set('5', appender5)
+	ss.Set('6', appender6)
+	ss.Set('7', appender7)
+	ss.Set('8', appender8)
+	ss.Set('9', appender9)
+
+	strftimeExtensions = strftime.WithSpecificationSet(ss)
+}
+
+// ================================================================
 // Argument 1 is formatted date string like "2021-03-04 02:59:50".
 // Argument 2 is format string like "%Y-%m-%d %H:%M:%S".
 func MlrvalStrptime(input1, input2 *Mlrval) *Mlrval {
@@ -106,6 +183,7 @@ func MlrvalStrptime(input1, input2 *Mlrval) *Mlrval {
 	return MlrvalPointerFromFloat64(float64(t.UnixNano()) / 1.0e9)
 }
 
+// ================================================================
 // Argument 1 is formatted date string like "2021-03-04T02:59:50Z".
 func MlrvalGMT2Sec(input1 *Mlrval) *Mlrval {
 	return MlrvalStrptime(input1, ptr_ISO8601_TIME_FORMAT)