fix: incorrect UTC conversion math

src/helpers/format-date.ts

@@ -104,8 +104,32 @@ export const formatNullableDateRange = (
   return startEnd;
 };
 
-export const dateSameAcrossTimezones = (date: Date): boolean => {
-  const endDayUserTZ = dayjs(date);
-  const endDayUTC = dayjs(date).utc(true);
-  return endDayUserTZ.isSame(endDayUTC);
+/**
+ * Formats a dayjs date in UTC timezone.
+ * This is needed because formatDate() uses toLocaleTimeString() which always
+ * formats in local timezone. This function formats directly in UTC using dayjs.
+ *
+ * Only shows the date if UTC falls on a different calendar day than local
+ * (e.g., "Today, 8pm PST Feb 6, 4am UTC"). If same day, just shows time
+ * (e.g., "Today, 10am PST 6pm UTC").
+ */
+export const formatDateAsUTC = (date: Dayjs): string => {
+  const utcDate = date.utc();
+  const localDate = date;
+
+  // Format time: "4pm" or "4:30pm"
+  const timeStr =
+    utcDate.minute() === 0 ? utcDate.format("ha") : utcDate.format("h:mma");
+
+  // Only show date if UTC and local fall on different calendar days
+  const sameCalendarDay =
+    utcDate.date() === localDate.date() &&
+    utcDate.month() === localDate.month() &&
+    utcDate.year() === localDate.year();
+
+  if (sameCalendarDay) {
+    return `${timeStr} UTC`;
+  }
+
+  return `${utcDate.format("MMM D")}, ${timeStr} UTC`;
 };

src/helpers/units.ts

@@ -5,7 +5,7 @@ import dayjs from "dayjs";
 import timezone from "dayjs/plugin/timezone";
 import utc from "dayjs/plugin/utc";
 import type { Nullable } from "../types/empty.ts";
-import { dateSameAcrossTimezones, formatDate } from "./format-date.ts";
+import { formatDate, formatDateAsUTC } from "./format-date.ts";
 
 dayjs.extend(utc);
 dayjs.extend(timezone);
@@ -158,11 +158,7 @@ export async function selectTime(
   const suggestedHigherUserTZ = `${formatDate(suggestedHigher.toDate(), {
     forceIncludeTime: true,
   })} ${dayjs(suggestedHigher).format("z")}`;
-  const suggestedHigherUTC = `${formatDate(suggestedHigher.utc(true).toDate(), {
-    today: suggestedHigher.toDate(),
-    showToday: dateSameAcrossTimezones(suggestedHigher.toDate()),
-    forceIncludeTime: true,
-  })} UTC`;
+  const suggestedHigherUTC = formatDateAsUTC(suggestedHigher);
 
   // If the lower boundary is in the past, suggest "NOW"
   if (suggestedLower.isBefore(dayjs(new Date()))) {
@@ -192,11 +188,7 @@ export async function selectTime(
     const suggestedLowerUserTZ = `${formatDate(suggestedLower.toDate(), {
       forceIncludeTime: true,
     })} ${dayjs(suggestedLower).format("z")}`;
-    const suggestedLowerUTC = `${formatDate(suggestedLower.utc(true).toDate(), {
-      today: suggestedLower.toDate(),
-      showToday: dateSameAcrossTimezones(suggestedLower.toDate()),
-      forceIncludeTime: true,
-    })} UTC`;
+    const suggestedLowerUTC = formatDateAsUTC(suggestedLower);
 
     const maxLength = Math.max(
       suggestedLowerUserTZ.length,