fix truncate in case of window offset

polars/polars-core/src/frame/groupby/dynamic.rs

@@ -156,7 +156,8 @@ impl DataFrame {
         }
 
         if options.truncate {
-            dt = dt.apply(|v| w.truncate(v));
+            let w = Window::new(options.every, options.period, options.offset);
+            dt = dt.apply(|v| w.truncate_no_offset(v));
         }
 
         if let (true, Some(lower), Some(higher)) =

polars/polars-time/src/window.rs

@@ -27,6 +27,11 @@ impl Window {
         self.every.truncate_nanoseconds(t) + self.offset
     }
 
+    #[inline]
+    pub fn truncate_no_offset(&self, t: TimeNanoseconds) -> TimeNanoseconds {
+        self.every.truncate_nanoseconds(t)
+    }
+
     /// returns the bounds for the earliest window bounds
     /// that contains the given time t.  For underlapping windows that
     /// do not contain time t, the window directly after time t will be returned.

py-polars/tests/test_datelike.py

@@ -242,3 +242,33 @@ def test_date_comp() -> None:
     a = pl.Series("a", [one, two])
     assert (a == one).to_list() == [True, False]
     assert (a == two).to_list() == [False, True]
+
+
+def test_truncate_negative_offset() -> None:
+    df = pl.DataFrame(
+        {
+            "event_date": [
+                datetime(2021, 4, 11),
+                datetime(2021, 4, 29),
+                datetime(2021, 5, 29),
+            ],
+            "adm1_code": [1, 2, 1],
+        }
+    )
+    out = df.groupby_dynamic(
+        time_column="event_date",
+        every="1mo",
+        period="2mo",
+        offset="-1mo",
+        include_boundaries=True,
+    ).agg(
+        [
+            pl.col("adm1_code"),
+        ]
+    )
+
+    assert out["event_date"].to_list() == [
+        datetime(2021, 4, 1),
+        datetime(2021, 4, 1),
+        datetime(2021, 5, 1),
+    ]