diff --git a/pyproject.toml b/pyproject.toml
index 0d227cf..a17a527 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -15,7 +15,6 @@ dependencies = [
 dev = [
     "pytest>=8.1.1",
     "mypy>=1.15.0",
-    "pytest>=8.3.5",
     "ruff>=0.11.2",
     "pymarkdownlnt>=0.9.25",
     "pre-commit>=4.2.0",
diff --git a/stapi-pydantic/pyproject.toml b/stapi-pydantic/pyproject.toml
index 8c4f1e8..e2f5d17 100644
--- a/stapi-pydantic/pyproject.toml
+++ b/stapi-pydantic/pyproject.toml
@@ -25,3 +25,8 @@ include = ["src/stapi_pydantic"]
 [build-system]
 requires = ["hatchling"]
 build-backend = "hatchling.build"
+
+[dependency-groups]
+dev = [
+    "pytest>=8.1.1",
+]
diff --git a/stapi-pydantic/src/stapi_pydantic/datetime_interval.py b/stapi-pydantic/src/stapi_pydantic/datetime_interval.py
index a581cf7..0139bb2 100644
--- a/stapi-pydantic/src/stapi_pydantic/datetime_interval.py
+++ b/stapi-pydantic/src/stapi_pydantic/datetime_interval.py
@@ -10,32 +10,37 @@
     WrapSerializer,
 )
 
+_DatetimeTuple = tuple[datetime | None, datetime | None]
 
-def validate_before(
-    value: str | tuple[datetime, datetime],
-) -> tuple[datetime, datetime]:
+
+def validate_before(value: str | _DatetimeTuple) -> _DatetimeTuple:
     if isinstance(value, str):
-        start, end = value.split("/", 1)
-        return (datetime.fromisoformat(start), datetime.fromisoformat(end))
+        start_str, end_str = value.split("/", 1)
+        start = None if start_str == ".." else datetime.fromisoformat(start_str)
+        end = None if end_str == ".." else datetime.fromisoformat(end_str)
+        value = (start, end)
     return value
 
 
-def validate_after(value: tuple[datetime, datetime]) -> tuple[datetime, datetime]:
-    if value[1] < value[0]:
+def validate_after(value: _DatetimeTuple) -> _DatetimeTuple:
+    # None/date & date/None are always valid
+    if value[1] and value[0] and value[1] < value[0]:
         raise ValueError("end before start")
     return value
 
 
 def serialize(
-    value: tuple[datetime, datetime],
-    serializer: Callable[[tuple[datetime, datetime]], tuple[str, str]],
+    value: _DatetimeTuple,
+    serializer: Callable[[_DatetimeTuple], tuple[str, str]],
 ) -> str:
     del serializer  # unused
-    return f"{value[0].isoformat()}/{value[1].isoformat()}"
+    start = value[0].isoformat() if value[0] else ".."
+    end = value[1].isoformat() if value[1] else ".."
+    return f"{start}/{end}"
 
 
 DatetimeInterval = Annotated[
-    tuple[AwareDatetime, AwareDatetime],
+    tuple[AwareDatetime | None, AwareDatetime | None],
     BeforeValidator(validate_before),
     AfterValidator(validate_after),
     WrapSerializer(serialize, return_type=str),
diff --git a/stapi-pydantic/tests/test_datetime_interval.py b/stapi-pydantic/tests/test_datetime_interval.py
new file mode 100644
index 0000000..01ee6c6
--- /dev/null
+++ b/stapi-pydantic/tests/test_datetime_interval.py
@@ -0,0 +1,21 @@
+import pytest
+from stapi_pydantic import DatetimeInterval
+
+
+def test_valid_datetime_intervals() -> None:
+    """Test the datetime interval validator."""
+    dt1 = DatetimeInterval.__metadata__[0].func("2025-04-01T00:00:00Z/2025-04-01T23:59:59Z")
+    _ = DatetimeInterval.__metadata__[1].func(dt1)
+    dt2 = DatetimeInterval.__metadata__[0].func("2025-04-01T00:00:00Z/..")
+    _ = DatetimeInterval.__metadata__[1].func(dt2)
+    dt3 = DatetimeInterval.__metadata__[0].func("../2025-04-01T23:59:59Z")
+    _ = DatetimeInterval.__metadata__[1].func(dt3)
+    dt4 = DatetimeInterval.__metadata__[0].func("../..")
+    _ = DatetimeInterval.__metadata__[1].func(dt4)
+
+
+def test_invalid_datetime_intervals() -> None:
+    """Test the datetime interval validator."""
+    with pytest.raises(ValueError, match="end before start"):
+        dt1 = DatetimeInterval.__metadata__[0].func("2025-04-01T00:00:00Z/2025-03-01T23:59:59Z")
+        _ = DatetimeInterval.__metadata__[1].func(dt1)
diff --git a/uv.lock b/uv.lock
index dc69de0..f16c05f 100644
--- a/uv.lock
+++ b/uv.lock
@@ -1553,7 +1553,6 @@ dev = [
     { name = "pygithub", specifier = ">=2.6.1" },
     { name = "pymarkdownlnt", specifier = ">=0.9.25" },
     { name = "pytest", specifier = ">=8.1.1" },
-    { name = "pytest", specifier = ">=8.3.5" },
     { name = "respx", specifier = ">=0.22.0" },
     { name = "ruff", specifier = ">=0.11.2" },
     { name = "types-click", specifier = ">=7.1.8" },
@@ -2152,12 +2151,20 @@ dependencies = [
     { name = "geojson-pydantic" },
 ]
 
+[package.dev-dependencies]
+dev = [
+    { name = "pytest" },
+]
+
 [package.metadata]
 requires-dist = [
     { name = "cql2", specifier = ">=0.3.6" },
     { name = "geojson-pydantic", specifier = ">=1.2.0" },
 ]
 
+[package.metadata.requires-dev]
+dev = [{ name = "pytest", specifier = ">=8.1.1" }]
+
 [[package]]
 name = "starlette"
 version = "0.46.2"