diff --git a/cmd/zenity/main.go b/cmd/zenity/main.go
index 2c3537c..4eb63fd 100644
--- a/cmd/zenity/main.go
+++ b/cmd/zenity/main.go
@@ -36,6 +36,7 @@ var (
 	questionDlg       bool
 	entryDlg          bool
 	listDlg           bool
+	calendarDlg       bool
 	passwordDlg       bool
 	fileSelectionDlg  bool
 	colorSelectionDlg bool
@@ -66,6 +67,11 @@ var (
 	columns    int
 	allowEmpty bool
 
+	// Calendar options
+	year  uint
+	month uint
+	day   uint
+
 	// File selection options
 	save             bool
 	directory        bool
@@ -145,6 +151,9 @@ func main() {
 			strResult(zenity.List(text, flag.Args(), opts...))
 		}
 
+	case calendarDlg:
+		calResult(zenity.Calendar(text, opts...))
+
 	case passwordDlg:
 		_, pw, err := zenity.Password(opts...)
 		strResult(pw, err)
@@ -181,6 +190,7 @@ func setupFlags() {
 	flag.BoolVar(&questionDlg, "question", false, "Display question dialog")
 	flag.BoolVar(&entryDlg, "entry", false, "Display text entry dialog")
 	flag.BoolVar(&listDlg, "list", false, "Display list dialog")
+	flag.BoolVar(&calendarDlg, "calendar", false, "Display calendar dialog")
 	flag.BoolVar(&passwordDlg, "password", false, "Display password dialog")
 	flag.BoolVar(&fileSelectionDlg, "file-selection", false, "Display file selection dialog")
 	flag.BoolVar(&colorSelectionDlg, "color-selection", false, "Display color selection dialog")
@@ -214,6 +224,12 @@ func setupFlags() {
 	flag.Bool("hide-header", true, "Hide the column headers")
 	flag.BoolVar(&allowEmpty, "allow-empty", true, "Allow empty selection (macOS only)")
 
+	// Calendar options
+	flag.UintVar(&year, "year", 0, "Set the calendar `year`")
+	flag.UintVar(&month, "month", 0, "Set the calendar `month`")
+	flag.UintVar(&day, "day", 0, "Set the calendar `day`")
+	flag.StringVar(&zenutil.DateFormat, "date-format", "%m/%d/%Y", "Set the `format` for the returned date")
+
 	// File selection options
 	flag.BoolVar(&save, "save", false, "Activate save mode")
 	flag.BoolVar(&directory, "directory", false, "Activate directory-only selection")
@@ -283,6 +299,9 @@ func validateFlags() {
 	if listDlg {
 		n++
 	}
+	if calendarDlg {
+		n++
+	}
 	if passwordDlg {
 		n++
 	}
@@ -341,13 +360,18 @@ func loadFlags() []zenity.Option {
 		setDefault(&text, "Select items from the list below:")
 		setDefault(&okLabel, "OK")
 		setDefault(&cancelLabel, "Cancel")
+	case calendarDlg:
+		setDefault(&title, "Calendar selection")
+		setDefault(&text, "Select a date from below:")
+		setDefault(&okLabel, "OK")
+		setDefault(&cancelLabel, "Cancel")
 	case passwordDlg:
 		setDefault(&title, "Type your password")
 		setDefault(&okLabel, "OK")
 		setDefault(&cancelLabel, "Cancel")
 	case progressDlg:
 		setDefault(&title, "Progress")
-		setDefault(&text, "Running...")
+		setDefault(&text, "Running…")
 		setDefault(&okLabel, "OK")
 		setDefault(&cancelLabel, "Cancel")
 	}
@@ -413,6 +437,18 @@ func loadFlags() []zenity.Option {
 		opts = append(opts, zenity.DisallowEmpty())
 	}
 
+	y, m, d := time.Now().Date()
+	if month != 0 {
+		m = time.Month(month)
+	}
+	if day != 0 {
+		d = int(day)
+	}
+	if year != 0 {
+		y = int(year)
+	}
+	opts = append(opts, zenity.DefaultDate(y, m, d))
+
 	// File selection options
 
 	if directory {
@@ -484,6 +520,12 @@ func lstResult(l []string, err error) {
 	os.Stdout.WriteString(zenutil.LineBreak)
 }
 
+func calResult(d time.Time, err error) {
+	errResult(err)
+	os.Stdout.WriteString(d.Format(zenutil.Strftime(zenutil.DateFormat)))
+	os.Stdout.WriteString(zenutil.LineBreak)
+}
+
 func colResult(c color.Color, err error) {
 	errResult(err)
 	os.Stdout.WriteString(zenutil.UnparseColor(c))
diff --git a/date_darwin.go b/date_darwin.go
index 082b14d..2de3344 100644
--- a/date_darwin.go
+++ b/date_darwin.go
@@ -10,7 +10,7 @@ func calendar(text string, opts options) (time.Time, error) {
 	var date zenutil.Date
 
 	date.OK, date.Cancel, date.Extra = getAlertButtons(opts)
-	date.Format = "yyyy-MM-dd"
+	date.Format = zenutil.StrftimeUTS35(zenutil.DateFormat)
 	if opts.time != nil {
 		date.Date = opts.time.Unix()
 	}
@@ -27,5 +27,5 @@ func calendar(text string, opts options) (time.Time, error) {
 	if err != nil {
 		return time.Time{}, err
 	}
-	return time.Parse("2006-01-02", str)
+	return time.Parse(zenutil.Strftime(zenutil.DateFormat), str)
 }
diff --git a/date_unix.go b/date_unix.go
index 51db210..366beb1 100644
--- a/date_unix.go
+++ b/date_unix.go
@@ -10,7 +10,7 @@ import (
 )
 
 func calendar(text string, opts options) (time.Time, error) {
-	args := []string{"--calendar", "--text", text, "--date-format=%F"}
+	args := []string{"--calendar", "--text", text, "--date-format", zenutil.DateFormat}
 	args = appendTitle(args, opts)
 	args = appendButtons(args, opts)
 	args = appendWidthHeight(args, opts)
@@ -27,5 +27,5 @@ func calendar(text string, opts options) (time.Time, error) {
 	if err != nil {
 		return time.Time{}, err
 	}
-	return time.Parse("2006-01-02", str)
+	return time.Parse(zenutil.Strftime(zenutil.DateFormat), str)
 }
diff --git a/internal/zenutil/date.go b/internal/zenutil/date.go
new file mode 100644
index 0000000..375603d
--- /dev/null
+++ b/internal/zenutil/date.go
@@ -0,0 +1,151 @@
+package zenutil
+
+import "strings"
+
+// https://strftime.org/
+var strftimeTable = 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",
+
+	'+': "Mon Jan _2 03:04:05 PM 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,
+}
+
+// Strftime is internal.
+func Strftime(fmt string) string {
+	var res strings.Builder
+	res.Grow(len(fmt))
+
+	const (
+		initial = iota
+		special
+	)
+
+	state := initial
+	for _, b := range []byte(fmt) {
+		switch state {
+		case initial:
+			if b == '%' {
+				state = special
+			} else {
+				res.WriteByte(b)
+				state = initial
+			}
+
+		case special:
+			s, ok := strftimeTable[b]
+			if ok {
+				res.WriteString(s)
+			} else {
+				res.WriteByte(b)
+			}
+			state = initial
+		}
+	}
+
+	return res.String()
+}
+
+// https://nsdateformatter.com/
+var strftimeUTS35Table = 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",
+
+	'+': "E MMM d hh:mm:ss a 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,
+}
+
+// StrftimeUTS35 is internal.
+func StrftimeUTS35(fmt string) string {
+	var res strings.Builder
+	res.Grow(len(fmt))
+
+	const (
+		initial = iota
+		special
+	)
+
+	state := initial
+	for _, b := range []byte(fmt) {
+		switch state {
+		case initial:
+			if b == '%' {
+				state = special
+			} else {
+				res.WriteByte(b)
+				state = initial
+			}
+
+		case special:
+			s, ok := strftimeUTS35Table[b]
+			if ok {
+				res.WriteString(s)
+			} else {
+				res.WriteByte(b)
+			}
+			state = initial
+		}
+	}
+
+	return res.String()
+}
diff --git a/internal/zenutil/env_darwin.go b/internal/zenutil/env_darwin.go
index 5d11472..c4fd4a5 100644
--- a/internal/zenutil/env_darwin.go
+++ b/internal/zenutil/env_darwin.go
@@ -2,8 +2,9 @@ package zenutil
 
 // These are internal.
 var (
-	Command   bool
-	Timeout   int
-	LineBreak = "\n"
-	Separator = "\x00"
+	Command    bool
+	Timeout    int
+	LineBreak  = "\n"
+	Separator  = "\x00"
+	DateFormat = "%F"
 )
diff --git a/internal/zenutil/env_unix.go b/internal/zenutil/env_unix.go
index 663d4e4..eaec7df 100644
--- a/internal/zenutil/env_unix.go
+++ b/internal/zenutil/env_unix.go
@@ -5,8 +5,9 @@ package zenutil
 
 // These are internal.
 var (
-	Command   bool
-	Timeout   int
-	LineBreak = "\n"
-	Separator = "\x1e"
+	Command    bool
+	Timeout    int
+	LineBreak  = "\n"
+	Separator  = "\x1e"
+	DateFormat = "%F"
 )
diff --git a/internal/zenutil/env_windows.go b/internal/zenutil/env_windows.go
index fdeed2b..993ea6c 100644
--- a/internal/zenutil/env_windows.go
+++ b/internal/zenutil/env_windows.go
@@ -2,8 +2,9 @@ package zenutil
 
 // These are internal.
 var (
-	Command   bool
-	Timeout   int
-	Separator string
-	LineBreak = "\r\n"
+	Command    bool
+	Timeout    int
+	Separator  string
+	LineBreak  = "\r\n"
+	DateFormat = "%F"
 )
diff --git a/internal/zenutil/unescape.go b/internal/zenutil/unescape.go
index e4a703e..e542960 100644
--- a/internal/zenutil/unescape.go
+++ b/internal/zenutil/unescape.go
@@ -1,5 +1,7 @@
 package zenutil
 
+import "strings"
+
 // Unescape is internal.
 func Unescape(s string) string {
 	// Apply rules described in:
@@ -12,7 +14,7 @@ func Unescape(s string) string {
 		escape3
 	)
 	var oct byte
-	var res []byte
+	var res strings.Builder
 	state := initial
 	for _, b := range []byte(s) {
 		switch state {
@@ -21,7 +23,7 @@ func Unescape(s string) string {
 			case '\\':
 				state = escape1
 			default:
-				res = append(res, b)
+				res.WriteByte(b)
 				state = initial
 			}
 
@@ -31,25 +33,25 @@ func Unescape(s string) string {
 				oct = b - '0'
 				state = escape2
 			case 'b':
-				res = append(res, '\b')
+				res.WriteByte('\b')
 				state = initial
 			case 'f':
-				res = append(res, '\f')
+				res.WriteByte('\f')
 				state = initial
 			case 'n':
-				res = append(res, '\n')
+				res.WriteByte('\n')
 				state = initial
 			case 'r':
-				res = append(res, '\r')
+				res.WriteByte('\r')
 				state = initial
 			case 't':
-				res = append(res, '\t')
+				res.WriteByte('\t')
 				state = initial
 			case 'v':
-				res = append(res, '\v')
+				res.WriteByte('\v')
 				state = initial
 			default:
-				res = append(res, b)
+				res.WriteByte(b)
 				state = initial
 			}
 
@@ -59,10 +61,11 @@ func Unescape(s string) string {
 				oct = oct<<3 | (b - '0')
 				state = escape3
 			case '\\':
-				res = append(res, oct)
+				res.WriteByte(oct)
 				state = escape1
 			default:
-				res = append(res, oct, b)
+				res.WriteByte(oct)
+				res.WriteByte(b)
 				state = initial
 			}
 
@@ -70,20 +73,21 @@ func Unescape(s string) string {
 			switch b {
 			case '0', '1', '2', '3', '4', '5', '6', '7':
 				oct = oct<<3 | (b - '0')
-				res = append(res, oct)
+				res.WriteByte(oct)
 				state = initial
 			case '\\':
-				res = append(res, oct)
+				res.WriteByte(oct)
 				state = escape1
 			default:
-				res = append(res, oct, b)
+				res.WriteByte(oct)
+				res.WriteByte(b)
 				state = initial
 			}
 		}
 	}
 	if state == escape2 || state == escape3 {
-		res = append(res, oct)
+		res.WriteByte(oct)
 	}
 
-	return string(res)
+	return res.String()
 }