chore: simplify Duration.is_constant_duration usage

crates/polars-plan/src/dsl/function_expr/temporal.rs

@@ -267,9 +267,7 @@ pub(super) fn date_offset(s: &[Series]) -> PolarsResult<Series> {
                 1 => match offsets.get(0) {
                     Some(offset) => {
                         let offset = Duration::parse(offset);
-                        tz.is_none()
-                            || tz.as_deref() == Some("UTC")
-                            || offset.is_constant_duration()
+                        offset.is_constant_duration(tz.as_deref())
                     },
                     None => false,
                 },

crates/polars-time/src/windows/duration.rs

@@ -32,7 +32,7 @@ pub struct Duration {
     months: i64,
     // the number of weeks for the duration
     weeks: i64,
-    // the number of nanoseconds for the duration
+    // the number of days for the duration
     days: i64,
     // the number of nanoseconds for the duration
     nsecs: i64,
@@ -363,16 +363,23 @@ impl Duration {
         self.nsecs == 0
     }
 
-    pub fn is_constant_duration(&self) -> bool {
-        self.months == 0 && self.weeks == 0 && self.days == 0
+    pub fn is_constant_duration(&self, time_zone: Option<&str>) -> bool {
+        if time_zone.is_none() || time_zone == Some("UTC") {
+            self.months == 0
+        } else {
+            // For non-native, non-UTC time zones, 1 calendar day is not
+            // necessarily 24 hours due to daylight savings time.
+            self.months == 0 && self.weeks == 0 && self.days == 0
+        }
+    }
     }
 
     /// Returns the nanoseconds from the `Duration` without the weeks or months part.
     pub fn nanoseconds(&self) -> i64 {
         self.nsecs
     }
 
-    /// Estimated duration of the window duration. Not a very good one if months != 0.
+    /// Estimated duration of the window duration. Not a very good one if not a constant duration.
     #[doc(hidden)]
     pub const fn duration_ns(&self) -> i64 {
         self.months * 28 * 24 * 3600 * NANOSECONDS