diff --git a/cmd/zenity/build.sh b/cmd/zenity/build.sh
index 4d3f7c5..cf568d6 100755
--- a/cmd/zenity/build.sh
+++ b/cmd/zenity/build.sh
@@ -18,7 +18,7 @@ go run github.com/randall77/makefat zenity zenity_macos_x64 zenity_macos_arm &&
 zip -9 zenity_macos.zip zenity
 
 zip -9 zenity_brew.zip zenity zenity.exe
-
 rm zenity zenity_macos_* zenity.exe
 
+GOOS=linux go build -tags dev
 go build -tags dev
diff --git a/cmd/zenity/main.go b/cmd/zenity/main.go
index 9257913..0e14bfe 100644
--- a/cmd/zenity/main.go
+++ b/cmd/zenity/main.go
@@ -19,6 +19,7 @@ import (
 	"time"
 
 	"github.com/ncruces/zenity"
+	"github.com/ncruces/zenity/internal/strftime"
 	"github.com/ncruces/zenity/internal/zenutil"
 )
 
@@ -522,7 +523,7 @@ func lstResult(l []string, err error) {
 
 func calResult(d time.Time, err error) {
 	errResult(err)
-	os.Stdout.WriteString(zenutil.Strftime(zenutil.DateFormat, d))
+	os.Stdout.WriteString(strftime.Format(zenutil.DateFormat, d))
 	os.Stdout.WriteString(zenutil.LineBreak)
 }
 
diff --git a/date_darwin.go b/date_darwin.go
index 11f6999..55edf18 100644
--- a/date_darwin.go
+++ b/date_darwin.go
@@ -3,16 +3,21 @@ package zenity
 import (
 	"time"
 
+	"github.com/ncruces/zenity/internal/strftime"
 	"github.com/ncruces/zenity/internal/zenutil"
 )
 
-func calendar(text string, opts options) (time.Time, error) {
+func calendar(text string, opts options) (t time.Time, err error) {
 	var date zenutil.Date
 
 	date.OK, date.Cancel, date.Extra = getAlertButtons(opts)
-	date.Format = zenutil.StrftimeUTS35(zenutil.DateFormat)
+	date.Format, err = strftime.UTS35(zenutil.DateFormat)
+	if err != nil {
+		return
+	}
 	if opts.time != nil {
-		date.Date = opts.time.Unix()
+		unix := opts.time.Unix()
+		date.Date = &unix
 	}
 
 	if opts.title != nil {
@@ -25,8 +30,7 @@ func calendar(text string, opts options) (time.Time, error) {
 	out, err := zenutil.Run(opts.ctx, "date", date)
 	str, err := strResult(opts, out, err)
 	if err != nil {
-		return time.Time{}, err
+		return
 	}
-	layout := zenutil.StrftimeLayout(zenutil.DateFormat)
-	return time.Parse(layout, str)
+	return strftime.Parse(zenutil.DateFormat, str)
 }
diff --git a/date_unix.go b/date_unix.go
index 80c8117..5e0ae42 100644
--- a/date_unix.go
+++ b/date_unix.go
@@ -6,6 +6,7 @@ import (
 	"strconv"
 	"time"
 
+	"github.com/ncruces/zenity/internal/strftime"
 	"github.com/ncruces/zenity/internal/zenutil"
 )
 
@@ -27,6 +28,5 @@ func calendar(text string, opts options) (time.Time, error) {
 	if err != nil {
 		return time.Time{}, err
 	}
-	layout := zenutil.StrftimeLayout(zenutil.DateFormat)
-	return time.Parse(layout, str)
+	return strftime.Parse(zenutil.DateFormat, str)
 }
diff --git a/internal/strftime/parser.go b/internal/strftime/parser.go
new file mode 100644
index 0000000..141f233
--- /dev/null
+++ b/internal/strftime/parser.go
@@ -0,0 +1,65 @@
+package strftime
+
+type parser struct {
+	fmt      string
+	specs    map[byte]string
+	unpadded map[byte]string
+	writeLit func(byte) error
+	writeFmt func(string) error
+	fallback func(spec byte, pad bool) error
+}
+
+func (p *parser) parse() error {
+	const (
+		initial = iota
+		specifier
+		nopadding
+	)
+
+	var err error
+	state := initial
+	for _, b := range []byte(p.fmt) {
+		switch state {
+		default:
+			if b == '%' {
+				state = specifier
+			} else {
+				err = p.writeLit(b)
+			}
+
+		case specifier:
+			if b == '-' {
+				state = nopadding
+				continue
+			}
+			if s, ok := p.specs[b]; ok {
+				err = p.writeFmt(s)
+			} else {
+				err = p.fallback(b, true)
+			}
+			state = initial
+
+		case nopadding:
+			if s, ok := p.unpadded[b]; ok {
+				err = p.writeFmt(s)
+			} else if s, ok := p.specs[b]; ok {
+				err = p.writeFmt(s)
+			} else {
+				err = p.fallback(b, false)
+			}
+			state = initial
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+
+	switch state {
+	case specifier:
+		return p.writeLit('%')
+	case nopadding:
+		return p.writeLit('-')
+	}
+	return nil
+}
diff --git a/internal/strftime/specifiers.go b/internal/strftime/specifiers.go
new file mode 100644
index 0000000..1caa9de
--- /dev/null
+++ b/internal/strftime/specifiers.go
@@ -0,0 +1,102 @@
+package strftime
+
+func (p *parser) goSpecifiers() {
+	// https://strftime.org/
+	p.specs = map[byte]string{
+		'B': "January",
+		'b': "Jan",
+		'h': "Jan",
+		'm': "01",
+		'A': "Monday",
+		'a': "Mon",
+		'e': "_2",
+		'd': "02",
+		'j': "002",
+		'H': "15",
+		'I': "03",
+		'M': "04",
+		'S': "05",
+		'Y': "2006",
+		'y': "06",
+		'p': "PM",
+		'Z': "MST",
+		'z': "-0700",
+		'L': "000",
+		'f': "000000",
+		'N': "000000000",
+
+		'+': "Mon Jan _2 15:04:05 MST 2006",
+		'c': "Mon Jan _2 15:04:05 2006",
+		'F': "2006-01-02",
+		'D': "01/02/06",
+		'x': "01/02/06",
+		'r': "03:04:05 PM",
+		'T': "15:04:05",
+		'X': "15:04:05",
+		'R': "15:04",
+
+		'%': "%",
+		't': "\t",
+		'n': "\n",
+	}
+
+	p.unpadded = map[byte]string{
+		'm': "1",
+		'd': "2",
+		'I': "3",
+		'M': "4",
+		'S': "5",
+	}
+}
+
+func (p *parser) uts35Specifiers() { // https://nsdateformatter.com/
+	p.specs = map[byte]string{
+		'B': "MMMM",
+		'b': "MMM",
+		'h': "MMM",
+		'm': "MM",
+		'A': "EEEE",
+		'a': "E",
+		'd': "dd",
+		'j': "DDD",
+		'H': "HH",
+		'I': "hh",
+		'M': "mm",
+		'S': "ss",
+		'Y': "yyyy",
+		'y': "yy",
+		'G': "YYYY",
+		'g': "YY",
+		'V': "ww",
+		'p': "a",
+		'Z': "zzz",
+		'z': "Z",
+		'L': "SSS",
+		'f': "SSSSSS",
+		'N': "SSSSSSSSS",
+
+		'+': "E MMM d HH:mm:ss zzz yyyy",
+		'c': "E MMM d HH:mm:ss yyyy",
+		'F': "yyyy-MM-dd",
+		'D': "MM/dd/yy",
+		'x': "MM/dd/yy",
+		'r': "hh:mm:ss a",
+		'T': "HH:mm:ss",
+		'X': "HH:mm:ss",
+		'R': "HH:mm",
+
+		'%': "%",
+		't': "\t",
+		'n': "\n",
+	}
+
+	p.unpadded = map[byte]string{
+		'm': "M",
+		'd': "d",
+		'j': "D",
+		'H': "H",
+		'I': "h",
+		'M': "m",
+		'S': "s",
+	}
+}
diff --git a/internal/strftime/strftime.go b/internal/strftime/strftime.go
new file mode 100644
index 0000000..88ea218
--- /dev/null
+++ b/internal/strftime/strftime.go
@@ -0,0 +1,148 @@
+package strftime
+
+import (
+	"bytes"
+	"errors"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func Format(fmt string, t time.Time) string {
+	var res strings.Builder
+
+	var parser parser
+	parser.fmt = fmt
+	parser.goSpecifiers()
+
+	parser.writeLit = res.WriteByte
+
+	parser.writeFmt = func(fmt string) error {
+		switch fmt {
+		default:
+			res.WriteString(t.Format(fmt))
+		case "000", "000000", "000000000":
+			res.WriteString(t.Format("." + fmt)[1:])
+		}
+		return nil
+	}
+
+	parser.fallback = func(spec byte, pad bool) error {
+		switch spec {
+		default:
+			return errors.New("strftime: unsupported specifier: %" + string(spec))
+		case 'C':
+			s := t.Format("2006")
+			res.WriteString(s[:len(s)-2])
+		case 'g':
+			y, _ := t.ISOWeek()
+			res.WriteString(time.Date(y, 1, 1, 0, 0, 0, 0, time.UTC).Format("06"))
+		case 'G':
+			y, _ := t.ISOWeek()
+			res.WriteString(time.Date(y, 1, 1, 0, 0, 0, 0, time.UTC).Format("2006"))
+		case 'V':
+			_, w := t.ISOWeek()
+			if w < 10 && pad {
+				res.WriteByte('0')
+			}
+			res.WriteString(strconv.Itoa(w))
+		case 'w':
+			w := int(t.Weekday())
+			res.WriteString(strconv.Itoa(w))
+		case 'u':
+			if w := int(t.Weekday()); w == 0 {
+				res.WriteByte('7')
+			} else {
+				res.WriteString(strconv.Itoa(w))
+			}
+		}
+		return nil
+	}
+
+	parser.parse()
+	return res.String()
+}
+
+func Parse(fmt, value string) (time.Time, error) {
+	layout, err := Layout(fmt)
+	if err != nil {
+		return time.Time{}, err
+	}
+	return time.Parse(layout, value)
+}
+
+func Layout(fmt string) (string, error) {
+	var res strings.Builder
+
+	var parser parser
+	parser.fmt = fmt
+	parser.goSpecifiers()
+
+	parser.writeLit = func(b byte) error {
+		if bytes.IndexByte([]byte("MonJan_0123456789"), b) >= 0 {
+			return errors.New("strftime: unsupported literal: " + string(b))
+		}
+		res.WriteByte(b)
+		return nil
+	}
+
+	parser.writeFmt = func(s string) error {
+		res.WriteString(s)
+		return nil
+	}
+
+	parser.fallback = func(spec byte, pad bool) error {
+		return errors.New("strftime: unsupported specifier: %" + string(spec))
+	}
+
+	if err := parser.parse(); err != nil {
+		return "", err
+	}
+
+	parser.writeFmt("")
+	return res.String(), nil
+}
+
+func UTS35(fmt string) (string, error) {
+	var parser parser
+	parser.fmt = fmt
+	parser.uts35Specifiers()
+
+	const quote = '\''
+	var literal bool
+	var res strings.Builder
+
+	parser.writeLit = func(b byte) error {
+		if b == quote {
+			res.WriteByte(quote)
+			res.WriteByte(quote)
+			return nil
+		}
+		if !literal && ('a' <= b && b <= 'z' || 'A' <= b && b <= 'Z') {
+			literal = true
+			res.WriteByte(quote)
+		}
+		res.WriteByte(b)
+		return nil
+	}
+
+	parser.writeFmt = func(s string) error {
+		if literal {
+			literal = false
+			res.WriteByte(quote)
+		}
+		res.WriteString(s)
+		return nil
+	}
+
+	parser.fallback = func(spec byte, pad bool) error {
+		return errors.New("strftime: unsupported specifier: %" + string(spec))
+	}
+
+	if err := parser.parse(); err != nil {
+		return "", err
+	}
+
+	parser.writeFmt("")
+	return res.String(), nil
+}
diff --git a/internal/zenutil/date.go b/internal/zenutil/date.go
deleted file mode 100644
index e540abe..0000000
--- a/internal/zenutil/date.go
+++ /dev/null
@@ -1,215 +0,0 @@
-package zenutil
-
-import (
-	"strings"
-	"time"
-)
-
-// Strftime is internal.
-func Strftime(fmt string, t time.Time) string {
-	var res strings.Builder
-	writeLit := res.WriteByte
-	writeFmt := func(fmt string) (int, error) {
-		return res.WriteString(t.Format(fmt))
-	}
-	strftimeGo(fmt, writeLit, writeFmt)
-	return res.String()
-}
-
-// StrftimeLayout is internal.
-func StrftimeLayout(fmt string) string {
-	var res strings.Builder
-	strftimeGo(fmt, res.WriteByte, res.WriteString)
-	return res.String()
-}
-
-func strftimeGo(fmt string, writeLit func(byte) error, writeFmt func(string) (int, error)) {
-	// https://strftime.org/
-	fmts := map[byte]string{
-		'B': "January",
-		'b': "Jan",
-		'h': "Jan",
-		'm': "01",
-		'A': "Monday",
-		'a': "Mon",
-		'e': "_2",
-		'd': "02",
-		'j': "002",
-		'H': "15",
-		'I': "03",
-		'M': "04",
-		'S': "05",
-		'Y': "2006",
-		'y': "06",
-		'p': "PM",
-		'Z': "MST",
-		'z': "-0700",
-		'L': "000",
-		'f': "000000",
-		'N': "000000000",
-
-		'+': "Mon Jan _2 15:04:05 MST 2006",
-		'c': "Mon Jan _2 15:04:05 2006",
-		'F': "2006-01-02",
-		'D': "01/02/06",
-		'x': "01/02/06",
-		'r': "03:04:05 PM",
-		'T': "15:04:05",
-		'X': "15:04:05",
-		'R': "15:04",
-
-		'%': "%",
-		't': "\t",
-		'n': LineBreak,
-	}
-
-	unpaded := map[byte]string{
-		'm': "1",
-		'd': "2",
-		'I': "3",
-		'M': "4",
-		'S': "5",
-	}
-
-	parser(fmt, fmts, unpaded, writeLit, writeFmt)
-}
-
-// StrftimeUTS35 is internal.
-func StrftimeUTS35(fmt string) string {
-	// https://nsdateformatter.com/
-	fmts := map[byte]string{
-		'B': "MMMM",
-		'b': "MMM",
-		'h': "MMM",
-		'm': "MM",
-		'A': "EEEE",
-		'a': "E",
-		'd': "dd",
-		'j': "DDD",
-		'H': "HH",
-		'I': "hh",
-		'M': "mm",
-		'S': "ss",
-		'Y': "yyyy",
-		'y': "yy",
-		'G': "YYYY",
-		'g': "YY",
-		'V': "ww",
-		'p': "a",
-		'Z': "zzz",
-		'z': "Z",
-		'L': "SSS",
-		'f': "SSSSSS",
-		'N': "SSSSSSSSS",
-
-		'+': "E MMM d HH:mm:ss zzz yyyy",
-		'c': "E MMM d HH:mm:ss yyyy",
-		'F': "yyyy-MM-dd",
-		'D': "MM/dd/yy",
-		'x': "MM/dd/yy",
-		'r': "hh:mm:ss a",
-		'T': "HH:mm:ss",
-		'X': "HH:mm:ss",
-		'R': "HH:mm",
-
-		'%': "%",
-		't': "\t",
-		'n': LineBreak,
-	}
-
-	unpaded := map[byte]string{
-		'm': "M",
-		'd': "d",
-		'j': "D",
-		'H': "H",
-		'I': "h",
-		'M': "m",
-		'S': "s",
-	}
-
-	const quote = '\''
-	var literal bool
-	var res strings.Builder
-
-	writeLit := func(b byte) error {
-		if b == quote {
-			res.WriteByte(quote)
-			return res.WriteByte(quote)
-		}
-		if !literal && ('a' <= b && b <= 'z' || 'A' <= b && b <= 'Z') {
-			literal = true
-			res.WriteByte(quote)
-		}
-		return res.WriteByte(b)
-	}
-
-	writeFmt := func(s string) (int, error) {
-		if literal {
-			literal = false
-			res.WriteByte(quote)
-		}
-		return res.WriteString(s)
-	}
-
-	parser(fmt, fmts, unpaded, writeLit, writeFmt)
-	writeFmt("")
-
-	return res.String()
-}
-
-func parser(
-	fmt string,
-	formats, unpadded map[byte]string,
-	writeLit func(byte) error, writeFmt func(string) (int, error)) {
-
-	const (
-		initial = iota
-		special
-		padding
-	)
-
-	state := initial
-	for _, b := range []byte(fmt) {
-		switch state {
-		case initial:
-			if b == '%' {
-				state = special
-			} else {
-				writeLit(b)
-			}
-
-		case special:
-			if b == '-' {
-				state = padding
-				continue
-			}
-			if s, ok := formats[b]; ok {
-				writeFmt(s)
-			} else {
-				writeLit('%')
-				writeLit(b)
-			}
-			state = initial
-
-		case padding:
-			if s, ok := unpadded[b]; ok {
-				writeFmt(s)
-			} else if s, ok := formats[b]; ok {
-				writeFmt(s)
-			} else {
-				writeLit('%')
-				writeLit('-')
-				writeLit(b)
-			}
-			state = initial
-		}
-	}
-
-	switch state {
-	case padding:
-		writeLit('%')
-		fallthrough
-	case special:
-		writeLit('-')
-	}
-}
diff --git a/internal/zenutil/osa_generated.go b/internal/zenutil/osa_generated.go
index a36961d..91dbdf6 100644
--- a/internal/zenutil/osa_generated.go
+++ b/internal/zenutil/osa_generated.go
@@ -29,8 +29,10 @@ ObjC.import('stdlib')
 var date=$.NSDatePicker.alloc.init
 date.setDatePickerStyle($.NSDatePickerStyleClockAndCalendar)
 date.setDatePickerElements($.NSDatePickerElementFlagYearMonthDay)
-date.setDateValue($.NSDate.dateWithTimeIntervalSince1970({{.Date}}))
 date.setFrameSize(date.fittingSize)
+{{- if .Date}}
+date.setDateValue($.NSDate.dateWithTimeIntervalSince1970({{.Date}}))
+{{- end}}
 var alert=$.NSAlert.alloc.init
 alert.setAccessoryView(date)
 alert.setMessageText({{json .Text}})
diff --git a/internal/zenutil/osascripts/date.gojs b/internal/zenutil/osascripts/date.gojs
index 76daed4..b52843f 100644
--- a/internal/zenutil/osascripts/date.gojs
+++ b/internal/zenutil/osascripts/date.gojs
@@ -9,8 +9,10 @@ ObjC.import('stdlib')
 var date = $.NSDatePicker.alloc.init
 date.setDatePickerStyle($.NSDatePickerStyleClockAndCalendar)
 date.setDatePickerElements($.NSDatePickerElementFlagYearMonthDay)
-date.setDateValue($.NSDate.dateWithTimeIntervalSince1970({{.Date}}))
 date.setFrameSize(date.fittingSize)
+{{- if .Date}}
+  date.setDateValue($.NSDate.dateWithTimeIntervalSince1970({{.Date}}))
+{{- end}}
 
 var alert = $.NSAlert.alloc.init
 alert.setAccessoryView(date)
diff --git a/internal/zenutil/run_darwin.go b/internal/zenutil/run_darwin.go
index c375cdd..d2bbfa0 100644
--- a/internal/zenutil/run_darwin.go
+++ b/internal/zenutil/run_darwin.go
@@ -220,7 +220,7 @@ type Progress struct {
 
 // Date is internal.
 type Date struct {
-	Date   int64
+	Date   *int64
 	Text   string
 	Info   string
 	Format string