diff --git a/frontend/static/day.js b/frontend/static/day.js
index c07aec3..53d271a 100644
--- a/frontend/static/day.js
+++ b/frontend/static/day.js
@@ -7,7 +7,7 @@ import { uv } from "./account.js";   // header sign-in entry + notification bell
 import { getJSON, TTL, prefetchViews } from "./cache.js";
 import { initFindButton, setFindLabel } from "./mappicker.js";
 import { TIER_COLORS, PRECIP_COLORS, DRY_COLOR, pctOrd, fmtPrecip, fmtWind, fmtHumid,
-         todayISO, weatherType, placeLabel } from "./shared.js";
+         todayISO, isoOfDate, weatherType, placeLabel } from "./shared.js";
 
 // Color a tier the same way the calendar cell would be colored for it.
 const tierColor = (c) => (c === "dry" ? DRY_COLOR : TIER_COLORS[c] || PRECIP_COLORS[c] || "");
@@ -44,7 +44,9 @@ function stepDay(delta) {
   if (!curDate) return;
   const d = new Date(curDate + "T00:00:00");
   d.setDate(d.getDate() + delta);
-  const iso = d.toISOString().slice(0, 10);
+  // Format in LOCAL time — curDate was parsed as local midnight, so toISOString()
+  // (UTC) would shift the result a day for UTC± viewers and desync the today guard.
+  const iso = isoOfDate(d);
   if (iso > todayISO()) return;   // don't step past today
   curDate = iso;
   fetchDay();
diff --git a/frontend/static/shared.js b/frontend/static/shared.js
index 423c5ea..b9afe5a 100644
--- a/frontend/static/shared.js
+++ b/frontend/static/shared.js
@@ -257,7 +257,10 @@ export const pctOrd = (n) => {
   if (!Number.isFinite(v)) return "—";
   return ord(Math.min(99, Math.max(1, Math.round(v))));
 };
-export const todayISO = () => new Date().toISOString().slice(0, 10);
+// Today in the viewer's LOCAL zone (see isoOfDate below). A UTC date rolls a day
+// early/late for UTC± viewers past local midnight, putting "today" and the
+// date-picker max out of reach of the day they're actually living in.
+export const todayISO = () => isoOfDate(new Date());
 export const esc = (s) => String(s).replace(/&/g, "&amp;").replace(/</g, "&lt;").replace(/>/g, "&gt;");
 // The heading label for a graded response: the resolved place name, or the
 // cell-center coordinates while (or if) no name resolves.