diff --git a/.github/workflows/main.yaml b/.github/workflows/main.yaml
index 973acf9c..328fd2e0 100644
--- a/.github/workflows/main.yaml
+++ b/.github/workflows/main.yaml
@@ -18,7 +18,7 @@ jobs:
           python-version: "3.8"
 
     runs-on: ${{ matrix.os }}
-
+    
     steps:
 
       #----------------------------------------------
diff --git a/linkml_runtime/utils/metamodelcore.py b/linkml_runtime/utils/metamodelcore.py
index f46b8d4a..6a184b34 100644
--- a/linkml_runtime/utils/metamodelcore.py
+++ b/linkml_runtime/utils/metamodelcore.py
@@ -3,9 +3,11 @@
 import re
 from dataclasses import field
 from decimal import Decimal
+import sys
 from typing import Union, Optional, Tuple
 from urllib.parse import urlparse
 
+import isodate
 from rdflib import Literal, BNode, URIRef
 from rdflib.namespace import is_ncname
 from rdflib.term import Identifier as rdflib_Identifier
@@ -230,13 +232,10 @@ def __new__(cls, value: Union[str, datetime.time, datetime.datetime, Literal]) -
             if not isinstance(value, datetime.time):
                 value = datetime.time.fromisoformat(value)
             return datetime.time.fromisoformat(str(value)).isoformat()
-        except TypeError as e:
-            pass
-        except ValueError as e:
-            pass
-        if not is_strict():
-            return str(value)
-        raise e
+        except (TypeError, ValueError) as e:
+            if is_strict():
+                raise e
+        return str(value)
 
     @classmethod
     def is_valid(cls, value: Union[str, datetime.time, datetime.datetime, Literal]) -> bool:
@@ -260,15 +259,15 @@ def __new__(cls, value: Union[str, datetime.date, Literal]) -> str:
             value = value.value
         try:
             if not isinstance(value, datetime.date):
-                value = datetime.date.fromisoformat(str(value))
+                if sys.version_info >= (3, 11):
+                    value = datetime.date.fromisoformat(str(value))
+                else:
+                    value = isodate.parse_date(value)
             return value.isoformat()
-        except TypeError as e:
-            pass
-        except ValueError as e:
-            pass
-        if not is_strict():
-            return str(value)
-        raise e
+        except (TypeError, ValueError) as e:
+            if is_strict():
+                raise e
+        return str(value)
 
     @classmethod
     def is_valid(cls, value: Union[str, datetime.date, Literal]) -> bool:
@@ -279,7 +278,10 @@ def is_valid(cls, value: Union[str, datetime.date, Literal]) -> bool:
         if not re.match(r'^\d{4}-\d{2}-\d{2}$', value):
             return False
         try:
-            datetime.date.fromisoformat(str(value))
+            if sys.version_info >= (3, 11):
+                datetime.date.fromisoformat(str(value))
+            else:
+                value = isodate.parse_date(value)
         except ValueError:
             return False
         return True
@@ -294,15 +296,18 @@ def __new__(cls, value: Union[str, datetime.datetime, Literal]) -> str:
             value = value.value
         try:
             if not isinstance(value, datetime.datetime):
-                value = datetime.datetime.fromisoformat(value)      # Note that this handles non 'T' format as well
+                if sys.version_info >= (3, 11):
+                    value = datetime.datetime.fromisoformat(value)      # Note that this handles non 'T' format as well
+                else:
+                    if "T" in str(value):
+                        value = isodate.parse_datetime(value)
+                    else:
+                        value = isodate.parse_datetime("T".join(value.strip().split(' ', 1)))
             return value.isoformat()
-        except TypeError as e:
-            pass
-        except ValueError as e:
-            pass
-        if not is_strict():
-            return str(value)
-        raise e
+        except (TypeError, ValueError) as e:
+            if is_strict():
+                raise e
+        return str(value)
 
     @classmethod
     def is_valid(cls, value: Union[str, datetime.datetime, Literal]) -> bool:
@@ -311,8 +316,16 @@ def is_valid(cls, value: Union[str, datetime.datetime, Literal]) -> bool:
         if isinstance(value, datetime.datetime):
             value = value.isoformat()
         try:
-            datetime.datetime.fromisoformat(value)
-        except ValueError:
+            if sys.version_info >= (3, 11):
+                datetime.datetime.fromisoformat(value)
+            else:
+                if "T" in str(value):
+                    isodate.parse_datetime(value)
+                elif " " in value.strip():
+                    isodate.parse_datetime("T".join(value.strip().split(' ', 1)))
+                else:
+                    datetime.datetime.fromisoformat(value)
+        except (ValueError, TypeError):
             return False
         return True
 
diff --git a/tests/test_utils/test_metamodelcore.py b/tests/test_utils/test_metamodelcore.py
index 1bc26739..f70b88d9 100644
--- a/tests/test_utils/test_metamodelcore.py
+++ b/tests/test_utils/test_metamodelcore.py
@@ -153,6 +153,7 @@ def test_time(self):
         XSDDate(datetime.datetime.now())
         self.assertFalse(XSDTime.is_valid('Jan 12, 2019'))
         self.assertFalse(XSDTime.is_valid(datetime.datetime.now()))
+        self.assertFalse(XSDTime.is_valid("2019-07-06T17:22:39Z"))
         self.assertTrue(XSDTime.is_valid(v))
 
     def test_date(self):
@@ -168,6 +169,9 @@ def test_date(self):
             XSDDate('Jan 12, 2019')
         with self.assertRaises(ValueError):
             XSDDate(datetime.datetime.now())
+        with self.assertRaises(ValueError):
+            XSDDate("2019-07-06T17:22:39Z")
+
         lax()
         bv = XSDDate('Jan 12, 2019')
         self.assertEqual('Jan 12, 2019', bv)
@@ -188,6 +192,7 @@ def test_datetime(self):
         vstr = str(Literal(v).value)
         self.assertEqual('2019-07-06 17:22:39.007300', vstr)       # Note that this has no 'T'
         self.assertEqual('2019-07-06T17:22:39.007300', XSDDateTime(vstr))
+        self.assertEqual('2019-07-06T17:22:39+00:00', XSDDateTime("2019-07-06T17:22:39Z"))
         with self.assertRaises(ValueError):
             XSDDateTime('Jan 12, 2019')
         lax()
diff --git a/tox.ini b/tox.ini
index d33deaa0..fa4cf1f4 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,12 +1,10 @@
 [tox]
-envlist = py37, py38, py39, py310
-isolated_build = true
-skipsdist = true
-
+envlist = py39, py310, py311, py312
 
 [testenv]
-whitelist_externals = poetry
-commands=
-    poetry install -v
-    poetry run python -m unittest
-    poetry run comparefiles --help
+skip_install = true
+allowlist_externals = poetry
+commands_pre =
+    poetry install
+commands =
+    poetry run pytest