diff --git a/cel-c/internal/timestampconv.cc b/cel-c/internal/timestampconv.cc
index 605b7a2..b6deb82 100644
--- a/cel-c/internal/timestampconv.cc
+++ b/cel-c/internal/timestampconv.cc
@@ -60,9 +60,16 @@ bool _cel_Timestamp_FromRFC3339(cel_Timestamp* cel_nonnull out,
     return false;
   }
 
-  absl::Duration duration = time - absl::UnixEpoch();
-  int64_t seconds = absl::IDivDuration(duration, absl::Seconds(1), &duration);
-  int32_t nanos = static_cast<int32_t>(absl::ToInt64Nanoseconds(duration));
+  // `absl::ParseTime` accepts "infinite-past" and "infinite-future" regardless
+  // of the format, neither of which are valid timestamps.
+  if (time == absl::InfinitePast() || time == absl::InfiniteFuture()) {
+    return false;
+  }
+  // `absl::ToUnixSeconds` rounds towards negative infinity, so the remainder is
+  // always in [0s, 1s), even for timestamps before the Unix epoch.
+  int64_t seconds = absl::ToUnixSeconds(time);
+  int32_t nanos = static_cast<int32_t>(
+      absl::ToInt64Nanoseconds(time - absl::FromUnixSeconds(seconds)));
 
   if (!cel_Timestamp_Valid(seconds, nanos)) {
     return false;
diff --git a/cel-c/internal/timestampconv_test.cc b/cel-c/internal/timestampconv_test.cc
index bdda8fc..627a367 100644
--- a/cel-c/internal/timestampconv_test.cc
+++ b/cel-c/internal/timestampconv_test.cc
@@ -141,6 +141,26 @@ TEST_F(CelTimestampConvTest, FromRFC3339Valid) {
   cel_Timestamp_ToUnix(ts, &seconds, &nanos);
   EXPECT_EQ(seconds, -1);
   EXPECT_EQ(nanos, 0);
+
+  // Pre-epoch timestamps with fractional seconds keep nanos non-negative.
+  EXPECT_TRUE(_cel_Timestamp_FromRFC3339(
+      &ts, cel_StringView_From("1969-12-31T23:59:58.5Z")));
+  cel_Timestamp_ToUnix(ts, &seconds, &nanos);
+  EXPECT_EQ(seconds, -2);
+  EXPECT_EQ(nanos, 500000000);
+
+  EXPECT_TRUE(_cel_Timestamp_FromRFC3339(
+      &ts, cel_StringView_From("0001-01-01T00:00:00.000000001Z")));
+  cel_Timestamp_ToUnix(ts, &seconds, &nanos);
+  EXPECT_EQ(seconds, cel_Timestamp_kMinSeconds);
+  EXPECT_EQ(nanos, 1);
+
+  // Sub-nanosecond precision is floored.
+  EXPECT_TRUE(_cel_Timestamp_FromRFC3339(
+      &ts, cel_StringView_From("1969-12-31T23:59:59.9999999999Z")));
+  cel_Timestamp_ToUnix(ts, &seconds, &nanos);
+  EXPECT_EQ(seconds, -1);
+  EXPECT_EQ(nanos, 999999999);
 }
 
 TEST_F(CelTimestampConvTest, ToCivilTime) {
@@ -251,6 +271,10 @@ TEST_F(CelTimestampConvTest, FromRFC3339Invalid) {
   cel_Timestamp ts;
   EXPECT_FALSE(_cel_Timestamp_FromRFC3339(&ts, cel_StringView_From("")));
   EXPECT_FALSE(_cel_Timestamp_FromRFC3339(&ts, cel_StringView_From("invalid")));
+  EXPECT_FALSE(
+      _cel_Timestamp_FromRFC3339(&ts, cel_StringView_From("infinite-past")));
+  EXPECT_FALSE(
+      _cel_Timestamp_FromRFC3339(&ts, cel_StringView_From("infinite-future")));
   EXPECT_FALSE(
       _cel_Timestamp_FromRFC3339(&ts, cel_StringView_From("2023-03-15")));
   EXPECT_FALSE(_cel_Timestamp_FromRFC3339(