make duration_* w/ monthly a lower bound

polars/polars-time/src/duration.rs

@@ -170,12 +170,12 @@ impl Duration {
     /// Estimated duration of the window duration. Not a very good one if months != 0.
     #[inline]
     pub const fn duration_ns(&self) -> i64 {
-        self.months * 30 * 24 * 3600 * NS_SECOND + self.nsecs
+        self.months * 28 * 24 * 3600 * NS_SECOND + self.nsecs
     }
 
     #[inline]
     pub const fn duration_ms(&self) -> i64 {
-        self.months * 30 * 24 * 3600 * MILLISECONDS + self.nsecs / 1_000_000
+        self.months * 28 * 24 * 3600 * MILLISECONDS + self.nsecs / 1_000_000
     }
 
     #[inline]

polars/polars-time/src/test.rs

@@ -29,6 +29,27 @@ fn test_date_range() {
     assert_eq!(dates, expected);
 }
 
+#[test]
+fn test_feb_date_range() {
+    let start = NaiveDate::from_ymd(2022, 2, 1).and_hms(0, 0, 0);
+    let end = NaiveDate::from_ymd(2022, 3, 1).and_hms(0, 0, 0);
+    let dates = date_range(
+        start.timestamp_nanos(),
+        end.timestamp_nanos(),
+        Duration::parse("1mo"),
+        ClosedWindow::Both,
+        TimeUnit::Nanoseconds,
+    );
+    let expected = [
+        NaiveDate::from_ymd(2022, 2, 1),
+        NaiveDate::from_ymd(2022, 3, 1),
+    ]
+    .iter()
+    .map(|d| d.and_hms(0, 0, 0).timestamp_nanos())
+    .collect::<Vec<_>>();
+    assert_eq!(dates, expected);
+}
+
 fn print_ns(ts: &[i64]) {
     for ts in ts {
         println!("{}", timestamp_ns_to_datetime(*ts));