3 files changed, 51 insertions, 3 deletions
diff --git a/src/encoding/asn1/asn1.go b/src/encoding/asn1/asn1.go
index 4e3f85de13..0b64f06d36 100644
--- a/src/encoding/asn1/asn1.go
+++ b/src/encoding/asn1/asn1.go
@@ -828,9 +828,18 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 	}
 
 	// Special case for time: UTCTime and GeneralizedTime both map to the
-	// Go type time.Time.
-	if universalTag == TagUTCTime && t.tag == TagGeneralizedTime && t.class == ClassUniversal {
-		universalTag = TagGeneralizedTime
+	// Go type time.Time. getUniversalType returns the tag for UTCTime when
+	// it sees a time.Time, so if we see a different time type on the wire,
+	// or the field is tagged with a different type, we change the universal
+	// type to match.
+	if universalTag == TagUTCTime {
+		if t.class == ClassUniversal {
+			if t.tag == TagGeneralizedTime {
+				universalTag = t.tag
+			}
+		} else if params.timeType != 0 {
+			universalTag = params.timeType
+		}
 	}
 
 	if params.set {
@@ -1103,6 +1112,13 @@ func setDefaultValue(v reflect.Value, params fieldParameters) (ok bool) {
 //	numeric causes strings to be unmarshaled as ASN.1 NumericString values
 //	utf8    causes strings to be unmarshaled as ASN.1 UTF8String values
 //
+// When decoding an ASN.1 value with an IMPLICIT tag into a time.Time field,
+// Unmarshal will default to a UTCTime, which doesn't support time zones or
+// fractional seconds. To force usage of GeneralizedTime, use the following
+// tag:
+//
+//	generalized causes time.Times to be unmarshaled as ASN.1 GeneralizedTime values
+//
 // If the type of the first field of a structure is RawContent then the raw
 // ASN1 contents of the struct will be stored in it.
 //
diff --git a/src/encoding/asn1/asn1_test.go b/src/encoding/asn1/asn1_test.go
index 60dae71df4..0597740bd5 100644
--- a/src/encoding/asn1/asn1_test.go
+++ b/src/encoding/asn1/asn1_test.go
@@ -1185,3 +1185,34 @@ func BenchmarkObjectIdentifierString(b *testing.B) {
 		_ = oidPublicKeyRSA.String()
 	}
 }
+
+func TestImplicitTypeRoundtrip(t *testing.T) {
+	type tagged struct {
+		IA5         string    `asn1:"tag:1,ia5"`
+		Printable   string    `asn1:"tag:2,printable"`
+		UTF8        string    `asn1:"tag:3,utf8"`
+		Numeric     string    `asn1:"tag:4,numeric"`
+		UTC         time.Time `asn1:"tag:5,utc"`
+		Generalized time.Time `asn1:"tag:6,generalized"`
+	}
+	a := tagged{
+		IA5:         "ia5",
+		Printable:   "printable",
+		UTF8:        "utf8",
+		Numeric:     "123 456",
+		UTC:         time.Now().UTC().Truncate(time.Second),
+		Generalized: time.Now().UTC().Truncate(time.Second),
+	}
+	enc, err := Marshal(a)
+	if err != nil {
+		t.Fatalf("Marshal failed: %s", err)
+	}
+	var b tagged
+	if _, err := Unmarshal(enc, &b); err != nil {
+		t.Fatalf("Unmarshal failed: %s", err)
+	}
+
+	if !reflect.DeepEqual(a, b) {
+		t.Fatalf("Unexpected diff after roundtripping struct\na: %#v\nb: %#v", a, b)
+	}
+}
diff --git a/src/encoding/asn1/marshal.go b/src/encoding/asn1/marshal.go
index b9c0b8bce0..70e4fafc12 100644
--- a/src/encoding/asn1/marshal.go
+++ b/src/encoding/asn1/marshal.go
@@ -725,6 +725,7 @@ func makeField(v reflect.Value, params fieldParameters) (e encoder, err error) {
 //	omitempty:   causes empty slices to be skipped
 //	printable:   causes strings to be marshaled as ASN.1, PrintableString values
 //	utf8:        causes strings to be marshaled as ASN.1, UTF8String values
+//	numeric:     causes strings to be marshaled as ASN.1, NumericString values
 //	utc:         causes time.Time to be marshaled as ASN.1, UTCTime values
 //	generalized: causes time.Time to be marshaled as ASN.1, GeneralizedTime values
 func Marshal(val any) ([]byte, error) {