diff --git a/docs/lib/arrow.md b/docs/lib/arrow.md
index 312ad0adc..ef7a31fee 100644
--- a/docs/lib/arrow.md
+++ b/docs/lib/arrow.md
@@ -60,6 +60,7 @@ Visualized with [Plot’s difference mark](https://observablehq.com/plot/marks/d
 
 ```js echo
 Plot.plot({
+  x: {type: "utc"},
   marks: [
     Plot.ruleY([0]),
     Plot.differenceY(table, {x: "date", y: "value"})
@@ -67,6 +68,12 @@ Plot.plot({
 })
 ```
 
+<div class="note">
+
+The chart above specifies _x_ as a UTC scale because Apache Arrow represents dates as numbers (milliseconds since [Unix epoch](<https://en.wikipedia.org/wiki/Epoch_(computing)>)) rather than Date objects; without this hint, Plot would assume that _date_ column is quantitative rather than temporal and produce a less legible axis.
+
+</div>
+
 ## Apache Parquet
 
 The [Apache Parquet](https://parquet.apache.org/) format is optimized for storage and transfer. To load a Parquet file — such as this sample of 250,000 stars from the [Gaia Star Catalog](https://observablehq.com/@cmudig/peeking-into-the-gaia-star-catalog) — use [`FileAttachment`](../files). This is implemented using Kyle Barron’s [parquet-wasm](https://kylebarron.dev/parquet-wasm/) library.