consistent week number parsing (#1807)

* convert week parsing to match d3.js
* corrected the test to show difference between iso and %W week numbering

src/fava/help/filters.md

@@ -20,6 +20,10 @@ current year up to today, or `year-1 - year` for all entries of the last and
 current year. To prevent subtraction, use parentheses: `(month)-10` refers to
 the 10th of this month, whereas `month-10` would be 10 months ago.
 
+**Week number** Week number of the year (Monday as the first day of the week) as
+a decimal number. All days in a new year preceding the first Monday are
+considered to be in week 0.
+
 ### Account
 
 Filter entries by account, matching any entry this account is part of. The

src/fava/util/date.py

@@ -389,7 +389,11 @@ def parse_date(  # noqa: PLR0911
     match = WEEK_RE.match(string)
     if match:
         year, week = map(int, match.group(1, 2))
-        start = datetime.date.fromisocalendar(year, week + 1, 1)
+        start = (
+            datetime.datetime.strptime(f"{year}-W{week}-1", "%Y-W%W-%w")
+            .replace(tzinfo=datetime.timezone.utc)
+            .date()
+        )
         return start, get_next_interval(start, Interval.WEEK)
 
     match = QUARTER_RE.match(string)

tests/test_util_date.py

@@ -220,6 +220,7 @@ def test_fiscal_substitute(
         ("2010-10-01", "2010-11-01", "2010-10"),
         ("2000-01-03", "2000-01-04", "2000-01-03"),
         ("2015-01-05", "2015-01-12", "2015-W01"),
+        ("2025-01-06", "2025-01-13", "2025-W01"),
         ("2015-04-01", "2015-07-01", "2015-Q2"),
         ("2014-01-01", "2016-01-01", "2014 to 2015"),
         ("2014-01-01", "2016-01-01", "2014-2015"),