Document older_than of ExpireSnapshots (apache#2324)

# Rationale for this change

I was looking into this, and took the liberty of changing the API to a
datetime rather than milliseconds to avoid anyone passing in seconds or
microseconds.


# Are these changes tested?

# Are there any user-facing changes?

<!-- In the case of user-facing changes, please add the changelog label.
-->

Author: Fokko

mkdocs/docs/api.md

@@ -1296,17 +1296,20 @@ PyIceberg provides table maintenance operations through the `table.maintenance`
 Expire old snapshots to clean up table metadata and reduce storage costs:
 
 ```python
-# Basic usage - expire a specific snapshot by ID
+# Expire snapshots older than three days
+from datetime import datetime, timedelta
+table.maintenance.expire_snapshots().older_than(
+    datetime.now() - timedelta(days=3)
+).commit()
+
+# Expire a specific snapshot by ID
 table.maintenance.expire_snapshots().by_id(12345).commit()
 
 # Context manager usage (recommended for multiple operations)
 with table.maintenance.expire_snapshots() as expire:
     expire.by_id(12345)
     expire.by_id(67890)
     # Automatically commits when exiting the context
-
-# Method chaining
-table.maintenance.expire_snapshots().by_id(12345).commit()
 ```
 
 #### Real-world Example

pyiceberg/table/update/snapshot.py

@@ -22,6 +22,7 @@
 from abc import abstractmethod
 from collections import defaultdict
 from concurrent.futures import Future
+from datetime import datetime
 from functools import cached_property
 from typing import TYPE_CHECKING, Callable, Dict, Generic, List, Optional, Set, Tuple
 
@@ -82,6 +83,7 @@
 )
 from pyiceberg.utils.bin_packing import ListPacker
 from pyiceberg.utils.concurrent import ExecutorFactory
+from pyiceberg.utils.datetime import datetime_to_millis
 from pyiceberg.utils.properties import property_as_bool, property_as_int
 
 if TYPE_CHECKING:
@@ -944,13 +946,11 @@ def _get_protected_snapshot_ids(self) -> Set[int]:
         Returns:
             Set of protected snapshot IDs to exclude from expiration.
         """
-        protected_ids: Set[int] = set()
-
-        for ref in self._transaction.table_metadata.refs.values():
-            if ref.snapshot_ref_type in [SnapshotRefType.TAG, SnapshotRefType.BRANCH]:
-                protected_ids.add(ref.snapshot_id)
-
-        return protected_ids
+        return {
+            ref.snapshot_id
+            for ref in self._transaction.table_metadata.refs.values()
+            if ref.snapshot_ref_type in [SnapshotRefType.TAG, SnapshotRefType.BRANCH]
+        }
 
     def by_id(self, snapshot_id: int) -> ExpireSnapshots:
         """
@@ -988,18 +988,19 @@ def by_ids(self, snapshot_ids: List[int]) -> "ExpireSnapshots":
             self.by_id(snapshot_id)
         return self
 
-    def older_than(self, timestamp_ms: int) -> "ExpireSnapshots":
+    def older_than(self, dt: datetime) -> "ExpireSnapshots":
         """
         Expire all unprotected snapshots with a timestamp older than a given value.
 
         Args:
-            timestamp_ms (int): Only snapshots with timestamp_ms < this value will be expired.
+            dt (datetime): Only snapshots with datetime < this value will be expired.
 
         Returns:
             This for method chaining.
         """
         protected_ids = self._get_protected_snapshot_ids()
+        expire_from = datetime_to_millis(dt)
         for snapshot in self._transaction.table_metadata.snapshots:
-            if snapshot.timestamp_ms < timestamp_ms and snapshot.snapshot_id not in protected_ids:
+            if snapshot.timestamp_ms < expire_from and snapshot.snapshot_id not in protected_ids:
                 self._snapshot_ids_to_expire.add(snapshot.snapshot_id)
         return self

tests/table/test_expire_snapshots.py

@@ -14,6 +14,7 @@
 # KIND, either express or implied.  See the License for the
 # specific language governing permissions and limitations
 # under the License.
+import datetime
 from unittest.mock import MagicMock
 from uuid import uuid4
 
@@ -142,7 +143,7 @@ def test_expire_snapshots_by_timestamp_skips_protected(table_v2: Table) -> None:
     table_v2.catalog = MagicMock()
 
     # Attempt to expire all snapshots before a future timestamp (so both are candidates)
-    future_timestamp = 9999999999999  # Far in the future, after any real snapshot
+    future_datetime = datetime.datetime.now() + datetime.timedelta(days=1)
 
     # Mock the catalog's commit_table to return the current metadata (simulate no change)
     mock_response = CommitTableResponse(
@@ -152,7 +153,7 @@ def test_expire_snapshots_by_timestamp_skips_protected(table_v2: Table) -> None:
     )
     table_v2.catalog.commit_table.return_value = mock_response
 
-    table_v2.maintenance.expire_snapshots().older_than(future_timestamp).commit()
+    table_v2.maintenance.expire_snapshots().older_than(future_datetime).commit()
     # Update metadata to reflect the commit (as in other tests)
     table_v2.metadata = mock_response.metadata