diff --git a/src/celpy/celtypes.py b/src/celpy/celtypes.py
index 29742a9..8f0bba0 100644
--- a/src/celpy/celtypes.py
+++ b/src/celpy/celtypes.py
@@ -1098,7 +1098,10 @@ def __repr__(self) -> str:
         return f"{self.__class__.__name__}({str(self)!r})"
 
     def __str__(self) -> str:
-        return self.strftime("%Y-%m-%dT%H:%M:%SZ")
+        text = self.strftime("%Y-%m-%dT%H:%M:%S%z")
+        if text.endswith("+0000"):
+            return f"{text[:-5]}Z"
+        return f"{text[:-2]}:{text[-2:]}"
 
     def __add__(self, other: Any) -> 'TimestampType':
         """Timestamp + Duration -> Timestamp"""
diff --git a/tests/test_celtypes.py b/tests/test_celtypes.py
index b89e4f4..05bbf02 100644
--- a/tests/test_celtypes.py
+++ b/tests/test_celtypes.py
@@ -339,6 +339,17 @@ def test_timestamp_type():
     assert ts_1.getSeconds() == IntType(30)
 
 
+def test_timestamp_type_issue_28():
+    utc = TimestampType('2020-10-20T12:00:00Z')
+    not_utc = TimestampType('2020-10-20T12:00:00-05:00')
+
+    assert repr(utc) == "TimestampType('2020-10-20T12:00:00Z')"
+    assert repr(not_utc) == "TimestampType('2020-10-20T12:00:00-05:00')"
+
+    assert utc != not_utc
+    assert str(utc) != str(not_utc)
+
+
 def test_extended_timestamp_type():
     others = {
         'et': 'US/Eastern',
diff --git a/type_check/lineprecision.txt b/type_check/lineprecision.txt
index 10930f0..6eafec4 100644
--- a/type_check/lineprecision.txt
+++ b/type_check/lineprecision.txt
@@ -5,7 +5,7 @@ celpy.__main__      465      172          7   42    244           0
 celpy.adapter       137       35          3    9     85           5
 celpy.c7nlib       1584      340         15  154   1075           0
 celpy.celparser     402      208          2   23    169           0
-celpy.celtypes     1500      435         14  221    791          39
+celpy.celtypes     1503      438         14  221    791          39
 celpy.evaluation   2471      839         33  176   1408          15
 xlate                 0        0          0    0      0           0
 xlate.c7n_to_cel   1730      387        102  145   1091           5