diff --git a/common/types/string.go b/common/types/string.go
index 5f5a4335..1335903a 100644
--- a/common/types/string.go
+++ b/common/types/string.go
@@ -122,7 +122,11 @@ func (s String) ConvertToType(typeVal ref.Type) ref.Val {
 			return durationOf(d)
 		}
 	case TimestampType:
-		if t, err := time.Parse(time.RFC3339, s.Value().(string)); err == nil {
+		str := s.Value().(string)
+		if !isStrictRFC3339(str) {
+			return NewErr("invalid RFC 3339 timestamp %q", str)
+		}
+		if t, err := time.Parse(time.RFC3339, str); err == nil {
 			if t.Unix() < minUnixTime || t.Unix() > maxUnixTime {
 				return celErrTimestampOverflow
 			}
diff --git a/common/types/string_test.go b/common/types/string_test.go
index 158f2bb7..daf8fdc5 100644
--- a/common/types/string_test.go
+++ b/common/types/string_test.go
@@ -15,6 +15,7 @@
 package types
 
 import (
+	"fmt"
 	"reflect"
 	"testing"
 	"time"
@@ -163,6 +164,50 @@ func TestStringConvertToType(t *testing.T) {
 	}
 }
 
+func TestStringConvertToTimestampStrict(t *testing.T) {
+	valid := []string{
+		"2025-01-17T01:00:00.001Z",
+		"2025-01-01T12:34:56Z",
+		"2025-01-01T12:34:56.123456789Z",
+		"2025-01-01T12:34:56+05:30",
+		"2025-01-01T12:34:56-08:00",
+		"2025-01-01T12:34:56+14:00",
+	}
+	for _, s := range valid {
+		if IsError(String(s).ConvertToType(TimestampType)) {
+			t.Errorf("String(%q).ConvertToType(TimestampType) errored, wanted a timestamp", s)
+		}
+	}
+	// RFC 3339 violations that time.Parse accepts loosely.
+	invalid := []string{
+		"2025-01-17T01:00:00,001Z",      // ',' fractional separator
+		"2025-01-17T1:00:00Z",           // single-digit hour
+		"2025-01-17T01:5:00Z",           // single-digit minute
+		"2025-01-18T01:01:01.001+24:01", // offset hour out of range
+		"2025-01-17T01:01:01.001+00:60", // offset minute out of range
+	}
+	for _, s := range invalid {
+		out := String(s).ConvertToType(TimestampType)
+		if !IsError(out) {
+			t.Errorf("String(%q).ConvertToType(TimestampType) succeeded, wanted an error", s)
+			continue
+		}
+		want := fmt.Sprintf("invalid RFC 3339 timestamp %q", s)
+		if got := out.(*Err).String(); got != want {
+			t.Errorf("String(%q).ConvertToType(TimestampType) errored with %q, wanted %q", s, got, want)
+		}
+	}
+}
+
+func BenchmarkStringConvertToTimestamp(b *testing.B) {
+	s := String("2025-01-01T12:34:56.123456789Z")
+	for i := 0; i < b.N; i++ {
+		if IsError(s.ConvertToType(TimestampType)) {
+			b.Fatal("ConvertToType(TimestampType) errored, wanted a timestamp")
+		}
+	}
+}
+
 func TestStringEqual(t *testing.T) {
 	if !String("hello").Equal(String("hello")).(Bool) {
 		t.Error("Two equivalent strings were not equal")
diff --git a/common/types/timestamp.go b/common/types/timestamp.go
index 060caf6b..8bdeab44 100644
--- a/common/types/timestamp.go
+++ b/common/types/timestamp.go
@@ -17,9 +17,11 @@ package types
 import (
 	"fmt"
 	"reflect"
+	"regexp"
 	"strconv"
 	"strings"
 	"time"
+	"unicode"
 
 	"github.com/google/cel-go/common/overloads"
 	"github.com/google/cel-go/common/types/ref"
@@ -52,6 +54,79 @@ const (
 	maxUnixTime int64 = 253402300799
 )
 
+// strictRFC3339Pattern gates the strings accepted by the `timestamp()` overload.
+// time.Parse accepts inputs that RFC 3339 forbids: a ',' fractional-second
+// separator, single-digit time fields, and numeric offsets whose hours exceed
+// 23 or minutes exceed 59. Those slip past unnoticed and shift the parsed
+// instant, so they are rejected before time.Parse runs. Month and day are held
+// to the grammar ranges 01-12 and 01-31; the remaining calendar validation
+// (day-of-month vs. month, leap years) is left to time.Parse.
+//
+// isStrictRFC3339 is the implementation used on the conversion path; the pattern
+// is retained as the reference the scan is conformance tested against.
+var strictRFC3339Pattern = regexp.MustCompile(
+	`^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])[Tt]([01]\d|2[0-3]):[0-5]\d:([0-5]\d|60)(\.\d+)?([Zz]|[+-]([01]\d|2[0-3]):[0-5]\d)$`)
+
+// isStrictRFC3339 reports whether s matches strictRFC3339Pattern, hand-rolled to
+// keep the conversion path off the regexp engine and its per-call cost.
+func isStrictRFC3339(s string) bool {
+	// Shortest accepted form is "2006-01-02T15:04:05Z" (20 bytes): a 19-byte
+	// fixed-width date-time followed by at least a 'Z'/'z' zone.
+	if len(s) < 20 {
+		return false
+	}
+	// full-date "T" partial-time
+	if !isYear(s[0:4]) || !isChar(s[4], '-') || !isMonth(s[5:7]) || !isChar(s[7], '-') || !isDay(s[8:10]) ||
+		!isChar(s[10], 't') ||
+		!isHour(s[11:13]) || !isChar(s[13], ':') || !isMinute(s[14:16]) || !isChar(s[16], ':') || !isSecond(s[17:19]) {
+		return false
+	}
+	rest := s[19:]
+	// optional fractional seconds: "." 1*DIGIT
+	if rest[0] == '.' {
+		rest = rest[1:]
+		n := 0
+		for n < len(rest) && isDigit(rest[n]) {
+			n++
+		}
+		if n == 0 {
+			return false
+		}
+		rest = rest[n:]
+	}
+	// time-offset: "Z" or ("+" / "-") time-hour ":" time-minute
+	if len(rest) == 1 {
+		return isChar(rest[0], 'z')
+	}
+	if len(rest) == 6 && (rest[0] == '+' || rest[0] == '-') {
+		return isHour(rest[1:3]) && isChar(rest[3], ':') && isMinute(rest[4:6])
+	}
+	return false
+}
+
+func isDigit(c byte) bool { return c >= '0' && c <= '9' }
+
+// isChar reports whether got is want, case-insensitively; want must be lower case.
+func isChar(got, want byte) bool {
+	g, w := rune(got), rune(want)
+	return g == w || unicode.ToLower(g) == w
+}
+
+// inRange reports whether s is all decimal digits and its value lies in [lo, hi].
+func inRange(s string, lo, hi uint64) bool {
+	u, err := strconv.ParseUint(s, 10, 64)
+	return err == nil && u >= lo && u <= hi
+}
+
+func isYear(s string) bool   { return inRange(s, 0, 9999) }
+func isMonth(s string) bool  { return inRange(s, 1, 12) }
+func isDay(s string) bool    { return inRange(s, 1, 31) }
+func isHour(s string) bool   { return inRange(s, 0, 23) }
+func isMinute(s string) bool { return inRange(s, 0, 59) }
+
+// isSecond permits 60 for a leap second.
+func isSecond(s string) bool { return inRange(s, 0, 60) }
+
 // Add implements traits.Adder.Add.
 func (t Timestamp) Add(other ref.Val) ref.Val {
 	switch other.Type() {
diff --git a/common/types/timestamp_test.go b/common/types/timestamp_test.go
index a7ff89d4..91f219a2 100644
--- a/common/types/timestamp_test.go
+++ b/common/types/timestamp_test.go
@@ -485,3 +485,49 @@ func TestTimestampGetMilliseconds(t *testing.T) {
 		t.Errorf("ts.getMilliseconds('America/Phoenix') got %v, wanted 1 ms", msTz)
 	}
 }
+
+func TestIsStrictRFC3339MatchesPattern(t *testing.T) {
+	// Exercise the hand-rolled scan against strictRFC3339Pattern, including the
+	// boundary cases for each field and a few well-formed timestamps. The two
+	// must agree on every input.
+	cases := []string{
+		"2025-01-01T12:34:56Z",
+		"2025-01-01T12:34:56z",
+		"2025-01-01t12:34:56Z",
+		"2025-01-01T12:34:56.123456789Z",
+		"2025-01-01T12:34:56.1Z",
+		"2025-01-01T00:00:00+00:00",
+		"2025-01-01T23:59:60-08:00",
+		"2025-01-01T12:34:56+14:00",
+		"2025-01-01T12:34:56+05:30",
+		"2025-01-01T20:00:00Z",
+		"2025-01-01T23:59:59Z",
+		"2025-12-31T12:34:56Z",
+		// rejected forms
+		"2025-00-01T12:34:56Z",
+		"2025-13-01T12:34:56Z",
+		"2025-01-00T12:34:56Z",
+		"2025-01-32T12:34:56Z",
+		"2025-01-01T12:34:56,123Z",
+		"2025-01-01T1:34:56Z",
+		"2025-01-01T12:3:56Z",
+		"2025-01-01T12:34:5Z",
+		"2025-01-01T24:00:00Z",
+		"2025-01-01T12:60:00Z",
+		"2025-01-01T12:34:61Z",
+		"2025-01-01T12:34:56.Z",
+		"2025-01-01T12:34:56+24:00",
+		"2025-01-01T12:34:56+00:60",
+		"2025-01-01T12:34:56+0530",
+		"2025-01-01T12:34:56",
+		"2025-01-01 12:34:56Z",
+		"2025-1-01T12:34:56Z",
+		"",
+		"not-a-timestamp",
+	}
+	for _, s := range cases {
+		if got, want := isStrictRFC3339(s), strictRFC3339Pattern.MatchString(s); got != want {
+			t.Errorf("isStrictRFC3339(%q) = %v, strictRFC3339Pattern.MatchString = %v", s, got, want)
+		}
+	}
+}