diff --git a/asn1time.go b/asn1time.go
new file mode 100644
index 0000000..7b66d4f
--- /dev/null
+++ b/asn1time.go
@@ -0,0 +1,209 @@
+package ctwatch
+
+import (
+	"time"
+	"strconv"
+	"errors"
+	"unicode"
+	"encoding/asn1"
+)
+
+func isDigit (b byte) bool {
+	return unicode.IsDigit(rune(b))
+}
+
+func bytesToInt (bytes []byte) (int, error) {
+	return strconv.Atoi(string(bytes))
+}
+
+func parseUTCTime (bytes []byte) (time.Time, error) {
+	var err error
+	var year, month, day int
+	var hour, min, sec int
+	var tz *time.Location
+
+	// YYMMDDhhmm
+	if len(bytes) < 10 {
+		return time.Time{}, errors.New("UTCTime is too short")
+	}
+	year, err = bytesToInt(bytes[0:2])
+	if err != nil { return time.Time{}, errors.New("UTCTime contains invalid integer: " + err.Error()) }
+
+	month, err = bytesToInt(bytes[2:4])
+	if err != nil { return time.Time{}, errors.New("UTCTime contains invalid integer: " + err.Error()) }
+
+	day, err = bytesToInt(bytes[4:6])
+	if err != nil { return time.Time{}, errors.New("UTCTime contains invalid integer: " + err.Error()) }
+
+	hour, err = bytesToInt(bytes[6:8])
+	if err != nil { return time.Time{}, errors.New("UTCTime contains invalid integer: " + err.Error()) }
+
+	min, err = bytesToInt(bytes[8:10])
+	if err != nil { return time.Time{}, errors.New("UTCTime contains invalid integer: " + err.Error()) }
+
+	bytes = bytes[10:]
+
+	// (optional) ss
+	if len(bytes) >= 2 && isDigit(bytes[0]) {
+		sec, err = bytesToInt(bytes[0:2])
+		if err != nil {
+			return time.Time{}, errors.New("UTCTime contains invalid integer: " + err.Error())
+		}
+		bytes = bytes[2:]
+	}
+
+	// timezone (required but allow it to be omitted, since this is a common error)
+	if len(bytes) >= 1 {
+		if bytes[0] == 'Z' {
+			tz = time.UTC
+			bytes = bytes[1:]
+		} else if bytes[0] == '+' {
+			// +hhmm
+			if len(bytes) < 5 {
+				return time.Time{}, errors.New("UTCTime positive timezone offset is too short")
+			}
+			tzHour, err := bytesToInt(bytes[1:3])
+			if err != nil { return time.Time{}, errors.New("UTCTime contains invalid integer: " + err.Error()) }
+
+			tzMin, err := bytesToInt(bytes[3:5])
+			if err != nil { return time.Time{}, errors.New("UTCTime contains invalid integer: " + err.Error()) }
+
+			tz = time.FixedZone("", tzHour * 3600 + tzMin * 60)
+			bytes = bytes[5:]
+		} else if bytes[0] == '-' {
+			// -hhmm
+			if len(bytes) < 5 {
+				return time.Time{}, errors.New("UTCTime negative timezone offset is too short")
+			}
+			tzHour, err := bytesToInt(bytes[1:3])
+			if err != nil { return time.Time{}, errors.New("UTCTime contains invalid integer: " + err.Error()) }
+
+			tzMin, err := bytesToInt(bytes[3:5])
+			if err != nil { return time.Time{}, errors.New("UTCTime contains invalid integer: " + err.Error()) }
+
+			tz = time.FixedZone("", -1 * (tzHour * 3600 + tzMin * 60))
+			bytes = bytes[5:]
+		}
+	} else {
+		tz = time.UTC
+	}
+
+	if len(bytes) > 0 {
+		return time.Time{}, errors.New("UTCTime has trailing garbage")
+	}
+
+	// https://tools.ietf.org/html/rfc5280#section-4.1.2.5.1
+	if year >= 50 {
+		year = 1900 + year
+	} else {
+		year = 2000 + year
+	}
+
+	return time.Date(year, time.Month(month), day, hour, min, sec, 0, tz), nil
+}
+
+func parseGeneralizedTime (bytes []byte) (time.Time, error) {
+	var err error
+	var year, month, day int
+	var hour, min, sec, ms int
+	var tz *time.Location
+
+	// YYYYMMDDHH
+	if len(bytes) < 10 {
+		return time.Time{}, errors.New("GeneralizedTime is too short")
+	}
+	year, err = bytesToInt(bytes[0:4])
+	if err != nil { return time.Time{}, errors.New("GeneralizedTime contains invalid integer: " + err.Error()) }
+
+	month, err = bytesToInt(bytes[4:6])
+	if err != nil { return time.Time{}, errors.New("GeneralizedTime contains invalid integer: " + err.Error()) }
+
+	day, err = bytesToInt(bytes[6:8])
+	if err != nil { return time.Time{}, errors.New("GeneralizedTime contains invalid integer: " + err.Error()) }
+
+	hour, err = bytesToInt(bytes[8:10])
+	if err != nil { return time.Time{}, errors.New("GeneralizedTime contains invalid integer: " + err.Error()) }
+
+	bytes = bytes[10:]
+
+	// (optional) MM
+	if len(bytes) >= 2 && isDigit(bytes[0]) {
+		min, err = bytesToInt(bytes[0:2])
+		if err != nil {
+			return time.Time{}, errors.New("GeneralizedTime contains invalid integer: " + err.Error())
+		}
+		bytes = bytes[2:]
+		// (optional) SS
+		if len(bytes) >= 2 && isDigit(bytes[0]) {
+			sec, err = bytesToInt(bytes[0:2])
+			if err != nil {
+				return time.Time{}, errors.New("GeneralizedTime contains invalid integer: " + err.Error())
+			}
+			bytes = bytes[2:]
+			// (optional) .fff
+			if len(bytes) >= 1 && bytes[0] == '.' {
+				if len(bytes) < 4 {
+					return time.Time{}, errors.New("GeneralizedTime fractional seconds is too short")
+				}
+				ms, err = bytesToInt(bytes[1:4])
+				if err != nil {
+					return time.Time{}, errors.New("GeneralizedTime contains invalid integer: " + err.Error())
+				}
+				bytes = bytes[4:]
+			}
+		}
+	}
+
+	// timezone (Z or +hhmm or -hhmm or nothing)
+	if len(bytes) >= 1 {
+		if bytes[0] == 'Z' {
+			bytes = bytes[1:]
+			tz = time.UTC
+		} else if bytes[0] == '+' {
+			// +hhmm
+			if len(bytes) < 5 {
+				return time.Time{}, errors.New("GeneralizedTime positive timezone offset is too short")
+			}
+			tzHour, err := bytesToInt(bytes[1:3])
+			if err != nil { return time.Time{}, errors.New("GeneralizedTime contains invalid integer: " + err.Error()) }
+
+			tzMin, err := bytesToInt(bytes[3:5])
+			if err != nil { return time.Time{}, errors.New("GeneralizedTime contains invalid integer: " + err.Error()) }
+
+			tz = time.FixedZone("", tzHour * 3600 + tzMin * 60)
+			bytes = bytes[5:]
+		} else if bytes[0] == '-' {
+			// -hhmm
+			if len(bytes) < 5 {
+				return time.Time{}, errors.New("GeneralizedTime negative timezone offset is too short")
+			}
+			tzHour, err := bytesToInt(bytes[1:3])
+			if err != nil { return time.Time{}, errors.New("GeneralizedTime contains invalid integer: " + err.Error()) }
+
+			tzMin, err := bytesToInt(bytes[3:5])
+			if err != nil { return time.Time{}, errors.New("GeneralizedTime contains invalid integer: " + err.Error()) }
+
+			tz = time.FixedZone("", -1 * (tzHour * 3600 + tzMin * 60))
+			bytes = bytes[5:]
+		}
+	} else {
+		tz = time.UTC
+	}
+
+	if len(bytes) > 0 {
+		return time.Time{}, errors.New("GeneralizedTime has trailing garbage")
+	}
+
+	return time.Date(year, time.Month(month), day, hour, min, sec, ms * 1000 * 1000, tz), nil
+}
+
+func decodeASN1Time (value *asn1.RawValue) (time.Time, error) {
+	if !value.IsCompound && value.Class == 0 {
+		if value.Tag == asn1.TagUTCTime {
+			return parseUTCTime(value.Bytes)
+		} else if value.Tag == asn1.TagGeneralizedTime {
+			return parseGeneralizedTime(value.Bytes)
+		}
+	}
+	return time.Time{}, errors.New("Not a time value")
+}
diff --git a/asn1time_test.go b/asn1time_test.go
new file mode 100644
index 0000000..58ad9f0
--- /dev/null
+++ b/asn1time_test.go
@@ -0,0 +1,113 @@
+package ctwatch
+
+import (
+	"testing"
+	"time"
+)
+
+type timeTest struct {
+	in	string
+	ok	bool
+	out	time.Time
+}
+
+var utcTimeTests = []timeTest{
+	{ "9502101525Z", true, time.Date(1995, time.February, 10, 15, 25, 0, 0, time.UTC) },
+	{ "950210152542Z", true, time.Date(1995, time.February, 10, 15, 25, 42, 0, time.UTC) },
+	{ "1502101525Z", true, time.Date(2015, time.February, 10, 15, 25, 0, 0, time.UTC) },
+	{ "150210152542Z", true, time.Date(2015, time.February, 10, 15, 25, 42, 0, time.UTC) },
+	{ "1502101525+1000", true, time.Date(2015, time.February, 10, 15, 25, 0, 0, time.FixedZone("", 10 * 3600)) },
+	{ "1502101525-1000", true, time.Date(2015, time.February, 10, 15, 25, 0, 0, time.FixedZone("", -1 * (10 * 3600))) },
+	{ "1502101525+1035", true, time.Date(2015, time.February, 10, 15, 25, 0, 0, time.FixedZone("", 10 * 3600 + 35 * 60)) },
+	{ "1502101525-1035", true, time.Date(2015, time.February, 10, 15, 25, 0, 0, time.FixedZone("", -1 * (10 * 3600 + 35 * 60))) },
+	{ "150210152542+1000", true, time.Date(2015, time.February, 10, 15, 25, 42, 0, time.FixedZone("", 10 * 3600)) },
+	{ "150210152542-1000", true, time.Date(2015, time.February, 10, 15, 25, 42, 0, time.FixedZone("", -1 * (10 * 3600))) },
+	{ "150210152542+1035", true, time.Date(2015, time.February, 10, 15, 25, 42, 0, time.FixedZone("", 10 * 3600 + 35 * 60)) },
+	{ "150210152542-1035", true, time.Date(2015, time.February, 10, 15, 25, 42, 0, time.FixedZone("", -1 * (10 * 3600 + 35 * 60))) },
+	{ "1502101525", true, time.Date(2015, time.February, 10, 15, 25, 0, 0, time.UTC) },
+	{ "150210152542", true, time.Date(2015, time.February, 10, 15, 25, 42, 0, time.UTC) },
+	{ "", false, time.Time{} },
+	{ "123", false, time.Time{} },
+	{ "150210152542-10", false, time.Time{} },
+	{ "150210152542F", false, time.Time{} },
+	{ "150210152542ZF", false, time.Time{} },
+}
+
+func TestUTCTime(t *testing.T) {
+	for i, test := range utcTimeTests {
+		ret, err := parseUTCTime([]byte(test.in))
+		if err != nil {
+			if test.ok {
+				t.Errorf("#%d: parseUTCTime(%q) failed with error %v", i, test.in, err)
+			}
+			continue
+		}
+		if !test.ok {
+			t.Errorf("#%d: parseUTCTime(%q) succeeded, should have failed", i, test.in)
+			continue
+		}
+		if !test.out.Equal(ret) {
+			t.Errorf("#%d: parseUTCTime(%q) = %v, want %v", i, test.in, ret, test.out)
+		}
+	}
+}
+
+var generalizedTimeTests = []timeTest{
+	{ "2015021015", true, time.Date(2015, time.February, 10, 15, 0, 0, 0, time.UTC) },
+	{ "201502101525", true, time.Date(2015, time.February, 10, 15, 25, 0, 0, time.UTC) },
+	{ "20150210152542", true, time.Date(2015, time.February, 10, 15, 25, 42, 0, time.UTC) },
+	{ "20150210152542.123", true, time.Date(2015, time.February, 10, 15, 25, 42, 123000000, time.UTC) },
+	{ "20150210152542.12", false, time.Time{} },
+	{ "20150210152542.1", false, time.Time{} },
+	{ "20150210152542.", false, time.Time{} },
+
+	{ "2015021015Z", true, time.Date(2015, time.February, 10, 15, 0, 0, 0, time.UTC) },
+	{ "201502101525Z", true, time.Date(2015, time.February, 10, 15, 25, 0, 0, time.UTC) },
+	{ "20150210152542Z", true, time.Date(2015, time.February, 10, 15, 25, 42, 0, time.UTC) },
+	{ "20150210152542.123Z", true, time.Date(2015, time.February, 10, 15, 25, 42, 123000000, time.UTC) },
+	{ "20150210152542.12Z", false, time.Time{} },
+	{ "20150210152542.1Z", false, time.Time{} },
+	{ "20150210152542.Z", false, time.Time{} },
+
+	{ "2015021015+1000", true, time.Date(2015, time.February, 10, 15, 0, 0, 0, time.FixedZone("", 10 * 3600)) },
+	{ "201502101525+1000", true, time.Date(2015, time.February, 10, 15, 25, 0, 0, time.FixedZone("", 10 * 3600)) },
+	{ "20150210152542+1000", true, time.Date(2015, time.February, 10, 15, 25, 42, 0, time.FixedZone("", 10 * 3600)) },
+	{ "20150210152542.123+1000", true, time.Date(2015, time.February, 10, 15, 25, 42, 123000000, time.FixedZone("", 10 * 3600)) },
+	{ "20150210152542.12+1000", false, time.Time{} },
+	{ "20150210152542.1+1000", false, time.Time{} },
+	{ "20150210152542.+1000", false, time.Time{} },
+
+	{ "2015021015-0835", true, time.Date(2015, time.February, 10, 15, 0, 0, 0, time.FixedZone("", -1 * (8 * 3600 + 35 * 60))) },
+	{ "201502101525-0835", true, time.Date(2015, time.February, 10, 15, 25, 0, 0, time.FixedZone("", -1 * (8 * 3600 + 35 * 60))) },
+	{ "20150210152542-0835", true, time.Date(2015, time.February, 10, 15, 25, 42, 0, time.FixedZone("", -1 * (8 * 3600 + 35 * 60))) },
+	{ "20150210152542.123-0835", true, time.Date(2015, time.February, 10, 15, 25, 42, 123000000, time.FixedZone("", -1 * (8 * 3600 + 35 * 60))) },
+	{ "20150210152542.12-0835", false, time.Time{} },
+	{ "20150210152542.1-0835", false, time.Time{} },
+	{ "20150210152542.-0835", false, time.Time{} },
+
+
+	{ "", false, time.Time{} },
+	{ "123", false, time.Time{} },
+	{ "2015021015+1000Z", false, time.Time{} },
+	{ "2015021015x", false, time.Time{} },
+	{ "201502101525Zf", false, time.Time{} },
+}
+
+func TestGeneralizedTime(t *testing.T) {
+	for i, test := range generalizedTimeTests {
+		ret, err := parseGeneralizedTime([]byte(test.in))
+		if err != nil {
+			if test.ok {
+				t.Errorf("#%d: parseGeneralizedTime(%q) failed with error %v", i, test.in, err)
+			}
+			continue
+		}
+		if !test.ok {
+			t.Errorf("#%d: parseGeneralizedTime(%q) succeeded, should have failed", i, test.in)
+			continue
+		}
+		if !test.out.Equal(ret) {
+			t.Errorf("#%d: parseGeneralizedTime(%q) = %v, want %v", i, test.in, ret, test.out)
+		}
+	}
+}
diff --git a/x509.go b/x509.go
index 45b0248..cc5484a 100644
--- a/x509.go
+++ b/x509.go
@@ -141,12 +141,25 @@ func ParseTBSCertificate (tbsBytes []byte) (*TBSCertificate, error) {
 }
 
 func (tbs *TBSCertificate) ParseValidity () (*CertValidity, error) {
-	var validity CertValidity
-	if rest, err := asn1.Unmarshal(tbs.Validity.FullBytes, &validity); err != nil {
+	var rawValidity struct {
+		NotBefore	asn1.RawValue
+		NotAfter	asn1.RawValue
+	}
+	if rest, err := asn1.Unmarshal(tbs.Validity.FullBytes, &rawValidity); err != nil {
 		return nil, errors.New("failed to parse validity: " + err.Error())
 	} else if len(rest) > 0 {
 		return nil, fmt.Errorf("trailing data after validity: %v", rest)
 	}
+
+	var validity CertValidity
+	var err error
+	if validity.NotBefore, err = decodeASN1Time(&rawValidity.NotBefore); err != nil {
+		return nil, errors.New("failed to decode notBefore time: " + err.Error())
+	}
+	if validity.NotAfter, err = decodeASN1Time(&rawValidity.NotAfter); err != nil {
+		return nil, errors.New("failed to decode notAfter time: " + err.Error())
+	}
+
 	return &validity, nil
 }