diff --git a/crates/augurs-prophet/src/features.rs b/crates/augurs-prophet/src/features.rs
index bde803b9..5e5fb23e 100644
--- a/crates/augurs-prophet/src/features.rs
+++ b/crates/augurs-prophet/src/features.rs
@@ -17,8 +17,8 @@ pub enum FeatureMode {
 #[derive(Debug, Clone)]
 pub struct Holiday {
     pub(crate) ds: Vec<TimestampSeconds>,
-    pub(crate) lower_window: Option<Vec<i32>>,
-    pub(crate) upper_window: Option<Vec<i32>>,
+    pub(crate) lower_window: Option<Vec<u32>>,
+    pub(crate) upper_window: Option<Vec<u32>>,
     pub(crate) prior_scale: Option<PositiveFloat>,
 }
 
@@ -37,9 +37,9 @@ impl Holiday {
     ///
     /// The lower window is the number of days before the holiday
     /// that it is observed. For example, if the holiday is on
-    /// 2023-01-01 and the lower window is -1, then the holiday will
+    /// 2023-01-01 and the lower window is 1, then the holiday will
     /// _also_ be observed on 2022-12-31.
-    pub fn with_lower_window(mut self, lower_window: Vec<i32>) -> Result<Self, Error> {
+    pub fn with_lower_window(mut self, lower_window: Vec<u32>) -> Result<Self, Error> {
         if self.ds.len() != lower_window.len() {
             return Err(Error::MismatchedLengths {
                 a_name: "ds".to_string(),
@@ -58,7 +58,7 @@ impl Holiday {
     /// that it is observed. For example, if the holiday is on
     /// 2023-01-01 and the upper window is 1, then the holiday will
     /// _also_ be observed on 2023-01-02.
-    pub fn with_upper_window(mut self, upper_window: Vec<i32>) -> Result<Self, Error> {
+    pub fn with_upper_window(mut self, upper_window: Vec<u32>) -> Result<Self, Error> {
         if self.ds.len() != upper_window.len() {
             return Err(Error::MismatchedLengths {
                 a_name: "ds".to_string(),
diff --git a/crates/augurs-prophet/src/prophet/prep.rs b/crates/augurs-prophet/src/prophet/prep.rs
index 5893c878..2fb5e2f5 100644
--- a/crates/augurs-prophet/src/prophet/prep.rs
+++ b/crates/augurs-prophet/src/prophet/prep.rs
@@ -664,12 +664,16 @@ impl<O> Prophet<O> {
             let lower = holiday
                 .lower_window
                 .as_ref()
-                .map(|x| Box::new(x.iter().copied()) as Box<dyn Iterator<Item = i32>>)
+                .map(|x| {
+                    Box::new(x.iter().copied().map(|x| x as i32)) as Box<dyn Iterator<Item = i32>>
+                })
                 .unwrap_or_else(|| Box::new(std::iter::repeat(0)));
             let upper = holiday
                 .upper_window
                 .as_ref()
-                .map(|x| Box::new(x.iter().copied()) as Box<dyn Iterator<Item = i32>>)
+                .map(|x| {
+                    Box::new(x.iter().copied().map(|x| x as i32)) as Box<dyn Iterator<Item = i32>>
+                })
                 .unwrap_or_else(|| Box::new(std::iter::repeat(0)));
 
             for (dt, lower, upper) in izip!(holiday.ds, lower, upper) {
@@ -678,7 +682,9 @@ impl<O> Prophet<O> {
                 let dt_date = dt - remainder;
 
                 // Check each of the possible offsets allowed by the lower/upper windows.
-                for offset in lower..=upper {
+                // We know that the lower window is always positive since it was originally
+                // a u32, so we can use `-lower..upper` here.
+                for offset in -lower..=upper {
                     let offset_seconds = offset as i64 * ONE_DAY_IN_SECONDS as i64;
                     let occurrence = dt_date + offset_seconds;
                     let col_name = FeatureName::Holiday {
diff --git a/js/augurs-prophet-js/src/lib.rs b/js/augurs-prophet-js/src/lib.rs
index 633c9e96..26dce638 100644
--- a/js/augurs-prophet-js/src/lib.rs
+++ b/js/augurs-prophet-js/src/lib.rs
@@ -1230,10 +1230,10 @@ pub struct Holiday {
     ///
     /// The lower window is the number of days before the holiday
     /// that it is observed. For example, if the holiday is on
-    /// 2023-01-01 and the lower window is -1, then the holiday will
+    /// 2023-01-01 and the lower window is 1, then the holiday will
     /// _also_ be observed on 2022-12-31.
     #[tsify(optional)]
-    pub lower_window: Option<Vec<i32>>,
+    pub lower_window: Option<Vec<u32>>,
 
     /// The upper window for the holiday.
     ///
@@ -1242,7 +1242,7 @@ pub struct Holiday {
     /// 2023-01-01 and the upper window is 1, then the holiday will
     /// _also_ be observed on 2023-01-02.
     #[tsify(optional)]
-    pub upper_window: Option<Vec<i32>>,
+    pub upper_window: Option<Vec<u32>>,
 
     /// The prior scale for the holiday.
     #[tsify(optional)]