Correct past and future directions for datetime durations

CHANGELOG

@@ -1,3 +1,9 @@
+Unreleased
+----------
+
+- Correct past and future directions for datetime values in
+  `format_timedelta(..., add_direction=True)` and the `timedeltaformat` filter.
+
 v3.1.0
 ------
 

flask_babel/__init__.py

@@ -445,14 +445,17 @@ def format_timedelta(
     add_direction=False,
     threshold=0.85,
 ):
-    """Format the elapsed time from the given date to now or the given
-    timedelta.
+    """Format the time between now and the given date or the given timedelta.
+
+    With ``add_direction=True``, future dates and positive timedeltas use
+    wording such as "in 1 hour"; past dates and negative timedeltas use
+    "1 hour ago".
 
     This function is also available in the template context as filter
     named `timedeltaformat`.
     """
     if isinstance(datetime_or_timedelta, datetime):
-        datetime_or_timedelta = datetime.utcnow() - datetime_or_timedelta
+        datetime_or_timedelta = datetime_or_timedelta - datetime.utcnow()
     return dates.format_timedelta(
         datetime_or_timedelta,
         granularity,

tests/test_date_formatting.py

@@ -1,11 +1,38 @@
 from datetime import datetime, timedelta
 
 import flask
+import pytest
 
 import flask_babel as babel
 from flask_babel import get_babel
 
 
+@pytest.mark.parametrize("hours, expected", [(-1, "1 hour ago"), (1, "in 1 hour")])
+@pytest.mark.parametrize("use_datetime", [True, False])
+def test_timedelta_direction(monkeypatch, hours, expected, use_datetime):
+    class FixedDatetime(datetime):
+        @classmethod
+        def utcnow(cls):
+            return cls(2026, 1, 1, 12)
+
+    monkeypatch.setattr(babel, "datetime", FixedDatetime)
+    app = flask.Flask(__name__)
+    babel.Babel(app)
+    value = timedelta(hours=hours)
+    if use_datetime:
+        value = FixedDatetime.utcnow() + value
+
+    with app.test_request_context():
+        assert babel.format_timedelta(value, add_direction=True) == expected
+        assert (
+            flask.render_template_string(
+                "{{ value|timedeltaformat(add_direction=True) }}", value=value
+            )
+            == expected
+        )
+        assert babel.format_timedelta(value) == "1 hour"
+
+
 def test_basics():
     app = flask.Flask(__name__)
     babel.Babel(app)