diff --git a/src/NATS.Server/JetStream/Storage/MemStore.cs b/src/NATS.Server/JetStream/Storage/MemStore.cs
index 71a347f..e80d1d5 100644
--- a/src/NATS.Server/JetStream/Storage/MemStore.cs
+++ b/src/NATS.Server/JetStream/Storage/MemStore.cs
@@ -132,7 +132,7 @@ public sealed class MemStore : IStreamStore
         lock (_gate)
         {
             var seq = _st.LastSeq + 1;
-            var ts = DateTime.UtcNow.Ticks * 100L; // nanos
+            var ts = ToUnixNanos(DateTime.UtcNow); // nanos
             StoreInternal(subject, null, payload.IsEmpty ? null : payload.ToArray(), seq, ts, 0);
             return ValueTask.FromResult(seq);
         }
@@ -149,7 +149,7 @@ public sealed class MemStore : IStreamStore
                 Sequence = m.Seq,
                 Subject = m.Subj,
                 Payload = m.Data ?? [],
-                TimestampUtc = new DateTime(m.Ts / 100L, DateTimeKind.Utc),
+                TimestampUtc = FromUnixNanos(m.Ts),
             });
         }
     }
@@ -167,7 +167,7 @@ public sealed class MemStore : IStreamStore
                 Sequence = m.Seq,
                 Subject = m.Subj,
                 Payload = m.Data ?? [],
-                TimestampUtc = new DateTime(m.Ts / 100L, DateTimeKind.Utc),
+                TimestampUtc = FromUnixNanos(m.Ts),
             });
         }
     }
@@ -183,7 +183,7 @@ public sealed class MemStore : IStreamStore
                     Sequence = m.Seq,
                     Subject = m.Subj,
                     Payload = m.Data ?? [],
-                    TimestampUtc = new DateTime(m.Ts / 100L, DateTimeKind.Utc),
+                    TimestampUtc = FromUnixNanos(m.Ts),
                 })
                 .ToArray();
             return ValueTask.FromResult<IReadOnlyList<StoredMessage>>(list);
@@ -219,7 +219,7 @@ public sealed class MemStore : IStreamStore
                     Sequence = x.Seq,
                     Subject = x.Subj,
                     PayloadBase64 = Convert.ToBase64String(x.Data ?? []),
-                    TimestampUtc = new DateTime(x.Ts / 100L, DateTimeKind.Utc),
+                    TimestampUtc = FromUnixNanos(x.Ts),
                 })
                 .ToArray();
             return ValueTask.FromResult(JsonSerializer.SerializeToUtf8Bytes(snapshot));
@@ -248,7 +248,7 @@ public sealed class MemStore : IStreamStore
                         var data = record.PayloadBase64.Length > 0
                             ? Convert.FromBase64String(record.PayloadBase64)
                             : null;
-                        var ts = new DateTimeOffset(record.TimestampUtc).ToUnixTimeMilliseconds() * 1_000_000L;
+                        var ts = ToUnixNanos(record.TimestampUtc);
                         // Advance LastSeq so StoreInternal accepts this sequence (handles gaps)
                         _st.LastSeq = record.Sequence - 1;
                         StoreInternal(record.Subject, null, data, record.Sequence, ts, 0);
@@ -290,7 +290,7 @@ public sealed class MemStore : IStreamStore
         {
             var seq = _st.LastSeq + 1;
             // Go uses time.Now().UnixNano() — use Ticks*100 for ~100ns precision
-            var ts = DateTime.UtcNow.Ticks * 100L;
+            var ts = ToUnixNanos(DateTime.UtcNow);
             StoreInternal(subject, hdr, msg.Length == 0 ? null : msg, seq, ts, ttl);
             return (seq, ts);
         }
@@ -563,7 +563,7 @@ public sealed class MemStore : IStreamStore
             }
             _st.LastSeq = seq;
             if (_msgs.TryGetValue(seq, out var last))
-                _st.LastTime = new DateTime(last.Ts / 100L, DateTimeKind.Utc);
+                _st.LastTime = FromUnixNanos(last.Ts);
         }
     }
 
@@ -576,7 +576,7 @@ public sealed class MemStore : IStreamStore
                 return _st.LastSeq + 1;
 
             // Go uses time.Time.UnixNano(); match with Ticks*100 (100ns resolution)
-            var ts = t.ToUniversalTime().Ticks * 100L;
+            var ts = ToUnixNanos(t.ToUniversalTime());
 
             if (!_msgs.TryGetValue(_st.FirstSeq, out var firstMsg))
                 return _st.LastSeq + 1;
@@ -878,7 +878,7 @@ public sealed class MemStore : IStreamStore
             seq = _st.LastSeq + 1;
         }
 
-        var now = new DateTime(ts / 100L, DateTimeKind.Utc);
+        var now = FromUnixNanos(ts);
         if (_st.Msgs == 0)
         {
             _st.FirstSeq = seq;
@@ -964,7 +964,7 @@ public sealed class MemStore : IStreamStore
         if (nsm != null)
         {
             _st.FirstSeq = nsm.Seq;
-            _st.FirstTime = new DateTime(nsm.Ts / 100L, DateTimeKind.Utc);
+            _st.FirstTime = FromUnixNanos(nsm.Ts);
         }
         else
         {
@@ -1062,7 +1062,7 @@ public sealed class MemStore : IStreamStore
                     if (_msgs.TryGetValue(newFirst, out var nm))
                     {
                         _st.FirstSeq = newFirst;
-                        _st.FirstTime = new DateTime(nm.Ts / 100L, DateTimeKind.Utc);
+                        _st.FirstTime = FromUnixNanos(nm.Ts);
                         break;
                     }
                 }
@@ -1165,6 +1165,12 @@ public sealed class MemStore : IStreamStore
         return size;
     }
 
+    // Go: time.Now().UnixNano() — convert DateTime to Unix nanoseconds
+    private static long ToUnixNanos(DateTime utc) => (utc.Ticks - DateTime.UnixEpoch.Ticks) * 100L;
+
+    // Convert Unix nanoseconds back to DateTime
+    private static DateTime FromUnixNanos(long nanos) => new(nanos / 100L + DateTime.UnixEpoch.Ticks, DateTimeKind.Utc);
+
     // Subject filter matching (same as FileStore helper)
     private static bool SubjectMatchesFilter(string subject, string filter)
     {
diff --git a/tests/NATS.Server.Tests/JetStream/Storage/StoreInterfaceTests.cs b/tests/NATS.Server.Tests/JetStream/Storage/StoreInterfaceTests.cs
index b64a571..a48aa98 100644
--- a/tests/NATS.Server.Tests/JetStream/Storage/StoreInterfaceTests.cs
+++ b/tests/NATS.Server.Tests/JetStream/Storage/StoreInterfaceTests.cs
@@ -426,8 +426,8 @@ public sealed class StoreInterfaceTests
         for (var seq = 4UL; seq <= 7UL; seq++)
             s.RemoveMsg(seq).ShouldBeTrue();
 
-        // Convert nanoseconds timestamp to DateTime.
-        var t = new DateTime(startTs / 100L, DateTimeKind.Utc);
+        // Convert Unix nanoseconds timestamp to DateTime.
+        var t = new DateTime(startTs / 100L + DateTime.UnixEpoch.Ticks, DateTimeKind.Utc);
         var found = s.GetSeqFromTime(t);
         found.ShouldBe(2UL);
     }
@@ -454,7 +454,7 @@ public sealed class StoreInterfaceTests
         // Delete last message — trailing delete.
         s.RemoveMsg(3).ShouldBeTrue();
 
-        var t = new DateTime(startTs / 100L, DateTimeKind.Utc);
+        var t = new DateTime(startTs / 100L + DateTime.UnixEpoch.Ticks, DateTimeKind.Utc);
         var found = s.GetSeqFromTime(t);
         found.ShouldBe(2UL);
     }