diff --git a/requirements.txt b/requirements.txt
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,4 +2,3 @@
 # should match https://pypi.python.org/pypi names. For the full spec or
 # dependency lines, see https://pip.readthedocs.org/en/1.1/requirements.html
 methodtools
-pytz
diff --git a/swh/provenance/provenance.py b/swh/provenance/provenance.py
--- a/swh/provenance/provenance.py
+++ b/swh/provenance/provenance.py
@@ -1,6 +1,5 @@
-from datetime import datetime
+from datetime import datetime, timezone
 import os
-import pytz
 from typing import Dict, Generator, List, Optional, Tuple
 
 from typing_extensions import Protocol, runtime_checkable
@@ -10,6 +9,8 @@
 from .origin import OriginEntry
 from .revision import RevisionEntry
 
+UTCMIN = datetime.min.replace(tzinfo=timezone.utc)
+
 
 @runtime_checkable
 class ProvenanceInterface(Protocol):
@@ -284,13 +285,12 @@
                         # Recursively analyse directory nodes.
                         stack.append(child)
             else:
-                maxdates = []
-                for child in current.children:
-                    assert child.maxdate is not None
-                    maxdates.append(child.maxdate)
-                current.maxdate = (
-                    max(maxdates) if maxdates else datetime.min.replace(tzinfo=pytz.UTC)
-                )
+                maxdates = [
+                    child.maxdate
+                    for child in current.children
+                    if child.maxdate is not None  # mostly to please mypy
+                ]
+                current.maxdate = max(maxdates) if maxdates else UTCMIN
         else:
             # Directory node in the frontier, just use its known date.
             current.maxdate = current.date
@@ -327,9 +327,7 @@
                 )
                 provenance.directory_add_to_revision(revision, current.entry, path)
                 directory_process_content(
-                    provenance,
-                    directory=current.entry,
-                    relative=current.entry,
+                    provenance, directory=current.entry, relative=current.entry,
                 )
             else:
                 # No point moving the frontier here. Either there are no files or they
diff --git a/swh/provenance/revision.py b/swh/provenance/revision.py
--- a/swh/provenance/revision.py
+++ b/swh/provenance/revision.py
@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timezone
 from itertools import islice
 import threading
 from typing import Iterable, Iterator, Optional, Tuple
@@ -20,6 +20,7 @@
         self.archive = archive
         self.id = id
         self.date = date
+        assert self.date is None or self.date.tzinfo is not None
         self.parents = parents
         self.root = root
 
@@ -78,11 +79,11 @@
     def __next__(self):
         with self.mutex:
             id, date, root = next(self.revisions)
+            date = datetime.fromisoformat(date)
+            if date.tzinfo is None:
+                date = date.replace(tzinfo=timezone.utc)
             return RevisionEntry(
-                self.archive,
-                hash_to_bytes(id),
-                date=datetime.fromisoformat(date),
-                root=hash_to_bytes(root),
+                self.archive, hash_to_bytes(id), date=date, root=hash_to_bytes(root),
             )