diff --git a/src/betterproto/__init__.py b/src/betterproto/__init__.py
index d1b0b99..5739b1f 100644
--- a/src/betterproto/__init__.py
+++ b/src/betterproto/__init__.py
@@ -1578,7 +1578,11 @@ class _Timestamp(Timestamp):
 
     def to_datetime(self) -> datetime:
         ts = self.seconds + (self.nanos / 1e9)
-        return datetime.fromtimestamp(ts, tz=timezone.utc)
+
+        if ts < 0:
+            return datetime(1970, 1, 1) + timedelta(seconds=ts)
+        else:
+            return datetime.fromtimestamp(ts, tz=timezone.utc)
 
     @staticmethod
     def timestamp_to_json(dt: datetime) -> str: