diff --git a/internal/strftime/specifiers.go b/internal/strftime/specifiers.go
index 1caa9de..7802b03 100644
--- a/internal/strftime/specifiers.go
+++ b/internal/strftime/specifiers.go
@@ -19,6 +19,7 @@ func (p *parser) goSpecifiers() {
 		'Y': "2006",
 		'y': "06",
 		'p': "PM",
+		'P': "pm",
 		'Z': "MST",
 		'z': "-0700",
 		'L': "000",
diff --git a/internal/strftime/strftime.go b/internal/strftime/strftime.go
index 1db0bd8..620e138 100644
--- a/internal/strftime/strftime.go
+++ b/internal/strftime/strftime.go
@@ -1,7 +1,6 @@
 package strftime
 
 import (
-	"bytes"
 	"errors"
 	"strconv"
 	"strings"
@@ -79,18 +78,32 @@ func Layout(fmt string) (string, error) {
 	parser.goSpecifiers()
 
 	parser.writeLit = func(b byte) error {
-		if bytes.IndexByte([]byte("MonJan_0123456789"), b) >= 0 {
-			return errors.New("strftime: unsupported literal: '" + string(b) + "'")
+		if '0' <= b && b <= '9' {
+			return errors.New("strftime: unsupported literal digit: '" + string(b) + "'")
 		}
 		res.WriteByte(b)
+		if b == 'M' || b == 'T' || b == 'm' || b == 'n' {
+			cur := res.String()
+			switch {
+			case strings.HasSuffix(cur, "Jan"):
+				return errors.New("strftime: unsupported literal: 'Jan'")
+			case strings.HasSuffix(cur, "Mon"):
+				return errors.New("strftime: unsupported literal: 'Mon'")
+			case strings.HasSuffix(cur, "MST"):
+				return errors.New("strftime: unsupported literal: 'MST'")
+			case strings.HasSuffix(cur, "PM"):
+				return errors.New("strftime: unsupported literal: 'PM'")
+			case strings.HasSuffix(cur, "pm"):
+				return errors.New("strftime: unsupported literal: 'pm'")
+			}
+		}
 		return nil
 	}
 
 	parser.writeFmt = func(fmt string) error {
 		switch fmt {
 		case "000", "000000", "000000000":
-			res := res.String()
-			if len := len(res); len <= 0 || res[len-1] != '.' && res[len-1] != ',' {
+			if cur := res.String(); !(strings.HasSuffix(cur, ".") || strings.HasSuffix(cur, ",")) {
 				return errors.New("strftime: unsupported specifier: fractional seconds must follow '.' or ','")
 			}
 		}
diff --git a/internal/strftime/strftime_test.go b/internal/strftime/strftime_test.go
new file mode 100644
index 0000000..4ba4e8a
--- /dev/null
+++ b/internal/strftime/strftime_test.go
@@ -0,0 +1,94 @@
+package strftime
+
+import (
+	"net/http"
+	"testing"
+	"time"
+)
+
+var reference = time.Date(2009, 8, 7, 6, 5, 4, 300000000, time.UTC)
+
+var timeTests = []struct {
+	format string
+	layout string
+	uts35  string
+	time   string
+}{
+	// Date and time formats
+	{"%c", time.ANSIC, "E MMM d HH:mm:ss yyyy", "Fri Aug  7 06:05:04 2009"},
+	{"%+", time.UnixDate, "E MMM d HH:mm:ss zzz yyyy", "Fri Aug  7 06:05:04 UTC 2009"},
+	{"%FT%TZ", time.RFC3339[:20], "yyyy-MM-dd'T'HH:mm:ss'Z'", "2009-08-07T06:05:04Z"},
+	{"%a %b %e %H:%M:%S %Y", time.ANSIC, "", "Fri Aug  7 06:05:04 2009"},
+	{"%a %b %e %H:%M:%S %Z %Y", time.UnixDate, "", "Fri Aug  7 06:05:04 UTC 2009"},
+	{"%a %b %d %H:%M:%S %z %Y", time.RubyDate, "E MMM dd HH:mm:ss Z yyyy", "Fri Aug 07 06:05:04 +0000 2009"},
+	{"%a, %d %b %Y %H:%M:%S %Z", time.RFC1123, "E, dd MMM yyyy HH:mm:ss zzz", "Fri, 07 Aug 2009 06:05:04 UTC"},
+	{"%a, %d %b %Y %H:%M:%S GMT", http.TimeFormat, "E, dd MMM yyyy HH:mm:ss 'GMT'", "Fri, 07 Aug 2009 06:05:04 GMT"},
+	{"%Y-%m-%dT%H:%M:%SZ", time.RFC3339[:20], "yyyy-MM-dd'T'HH:mm:ss'Z'", "2009-08-07T06:05:04Z"},
+	// Date formats
+	{"%F", "2006-01-02", "yyyy-MM-dd", "2009-08-07"},
+	{"%D", "01/02/06", "MM/dd/yy", "08/07/09"},
+	{"%x", "01/02/06", "MM/dd/yy", "08/07/09"},
+	{"%Y-%m-%d", "2006-01-02", "yyyy-MM-dd", "2009-08-07"},
+	{"%m/%d/%y", "01/02/06", "MM/dd/yy", "08/07/09"},
+	// Time formats
+	{"%R", "15:04", "HH:mm", "06:05"},
+	{"%T", "15:04:05", "HH:mm:ss", "06:05:04"},
+	{"%X", "15:04:05", "HH:mm:ss", "06:05:04"},
+	{"%r", "03:04:05 PM", "hh:mm:ss a", "06:05:04 AM"},
+	{"%H:%M", "15:04", "HH:mm", "06:05"},
+	{"%H:%M:%S", "15:04:05", "HH:mm:ss", "06:05:04"},
+	{"%I:%M:%S %p", "03:04:05 PM", "hh:mm:ss a", "06:05:04 AM"},
+	// Misc
+	{"%g", "", "YY", "09"},
+	{"%V/%G", "", "ww/YYYY", "32/2009"},
+	{"%Cth Century Fox", "", "", "20th Century Fox"},
+	{"%-I o'clock", "3 o'clock", "h 'o''clock'", "6 o'clock"},
+	{"%-A, the %uth day of the week", "", "", "Friday, the 5th day of the week"},
+	{"%Nns since %T", "", "SSSSSSSSS'ns since 'HH:mm:ss", "300000000ns since 06:05:04"},
+	{"%-S.%Ls since %R", "5.000s since 15:04", "s.SSS's since 'HH:mm", "4.300s since 06:05"},
+	// Parsing
+	{"", "", "", ""},
+	{"%", "%", "%", "%"},
+	{"%-", "-", "-", "-"},
+	{"%n", "\n", "\n", "\n"},
+	{"%t", "\t", "\t", "\t"},
+	{"%q", "", "", ""},
+	{"%-q", "", "", ""},
+	{"'", "'", "''", "'"},
+	{"100%", "", "100%", "100%"},
+	{"Monday", "", "'Monday'", "Monday"},
+	{"January", "", "'January'", "January"},
+	{"MST", "", "'MST'", "MST"},
+	{"AM", "AM", "'AM'", "AM"},
+	{"am", "am", "'am'", "am"},
+	{"PM", "", "'PM'", "PM"},
+	{"pm", "", "'pm'", "pm"},
+}
+
+func TestFormat(t *testing.T) {
+	for _, test := range timeTests {
+		if got := Format(test.format, reference); got != test.time {
+			t.Errorf("Format(%q) = %q, want %q", test.format, got, test.time)
+		}
+	}
+}
+
+func TestLayout(t *testing.T) {
+	for _, test := range timeTests {
+		if got, err := Layout(test.format); err != nil && test.layout != "" {
+			t.Errorf("Layout(%q) = %v", test.format, err)
+		} else if got != test.layout {
+			t.Errorf("Layout(%q) = %q, want %q", test.format, got, test.layout)
+		}
+	}
+}
+
+func TestUTS35(t *testing.T) {
+	for _, test := range timeTests {
+		if got, err := UTS35(test.format); err != nil && test.uts35 != "" {
+			t.Errorf("UTS35(%q) = %v", test.format, err)
+		} else if got != test.uts35 {
+			t.Errorf("UTS35(%q) = %q, want %q", test.format, got, test.uts35)
+		}
+	}
+}