fix(scheduled-tasks): use local-time date parser for calendar anchor (avoid UTC day-shift)

Author: waleedlatif1

.claude/rules/sim-url-state.md

@@ -164,9 +164,13 @@ export const thingsParsers = {
 
 Both carry the shared filter options (`{ history: 'replace', clearOnDefault: true }`). The defaults must match the list's existing default sort exactly. If a UI exposes "no active sort" as `null`, derive that in the component (`sort === DEFAULT && dir === DEFAULT ? null : { column, direction }`) — the URL still holds the resolved values. "Clear sort" writes the defaults back (which `clearOnDefault` strips from the URL); never write `null`/garbage columns.
 
-## Dates in the URL (`parseAsIsoDate`)
+## Dates in the URL (date-only params)
 
-A date-only param (a calendar anchor, a date filter) uses the built-in `parseAsIsoDate` (`yyyy-MM-dd`) — never serialize a full `Date`/timestamp when only the day matters. When the default is **dynamic** (e.g. "today"), make the param **nullable** (omit `.withDefault`) and derive the fallback in the hook (`const anchor = param ?? today`), so a clean URL means the dynamic default and navigating back to it writes `null` (clears the param). See `scheduled-tasks/search-params.ts` + `hooks/use-calendar.ts`.
+A date-only param (a calendar anchor, a date filter) is stored as `yyyy-MM-dd` — never serialize a full `Date`/timestamp when only the day matters.
+
+**Local vs UTC — pick the parser that matches your date math.** nuqs's built-in `parseAsIsoDate` is **UTC-based** (`serialize` via `toISOString()`, `parse` to UTC midnight). If your `Date` is local-time (e.g. produced by local-time helpers and read by `date-fns` `startOfWeek`/`isSameDay`, which are all local), `parseAsIsoDate` will shift the day by ±1 in any non-UTC timezone on reload/deep-link/back-forward. For local-time date math, use a small local-date `createParser` that serializes/parses on local calendar fields (`getFullYear`/`getMonth`/`getDate` ↔ `new Date(y, m-1, d)`) with an `eq` comparing y/m/d. Only use `parseAsIsoDate` when the value is genuinely UTC/midnight-UTC. See `scheduled-tasks/search-params.ts` (`parseAsLocalDate`).
+
+When the default is **dynamic** (e.g. "today"), make the param **nullable** (omit `.withDefault`) and derive the fallback in the hook (`const anchor = param ?? today`), so a clean URL means the dynamic default and navigating back to it writes `null` (clears the param). See `scheduled-tasks/hooks/use-calendar.ts`.
 
 ## Selected-entity deep-link (store the id, derive the object)
 

apps/sim/app/workspace/[workspaceId]/scheduled-tasks/search-params.ts

@@ -1,25 +1,57 @@
-import { parseAsIsoDate, parseAsStringLiteral } from 'nuqs/server'
+import { createParser, parseAsStringLiteral } from 'nuqs/server'
 
 const CALENDAR_SCOPES = ['day', 'week', 'month'] as const
 
 /** Default calendar granularity; matches the prior `useState` initial scope. */
 export const DEFAULT_CALENDAR_SCOPE = 'week'
 
+const pad2 = (n: number) => String(n).padStart(2, '0')
+
+/**
+ * Local-time date-only parser (`yyyy-MM-dd`).
+ *
+ * Unlike nuqs's built-in `parseAsIsoDate` — which serializes via `toISOString()`
+ * and parses to **UTC** midnight — this reads and writes the date using the
+ * browser's **local** calendar fields. The calendar's `anchor` is a local-time
+ * `Date` (`zonedClockDate`) and all the grid math (`date-fns`) is local, so a
+ * UTC-based parser shifts the day by ±1 in any non-UTC timezone on
+ * reload/deep-link/back-forward. This local parser round-trips losslessly against
+ * that local-time math.
+ */
+const parseAsLocalDate = createParser<Date>({
+  parse(value) {
+    const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(value)
+    if (!match) return null
+    const date = new Date(Number(match[1]), Number(match[2]) - 1, Number(match[3]))
+    return Number.isNaN(date.getTime()) ? null : date
+  },
+  serialize(value) {
+    return `${value.getFullYear()}-${pad2(value.getMonth() + 1)}-${pad2(value.getDate())}`
+  },
+  eq(a, b) {
+    return (
+      a.getFullYear() === b.getFullYear() &&
+      a.getMonth() === b.getMonth() &&
+      a.getDate() === b.getDate()
+    )
+  },
+})
+
 /**
  * Co-located, typed URL query-param definitions for the scheduled-tasks calendar.
  *
  * - `scope` is the calendar granularity (`day` / `week` / `month`).
- * - `anchor` is the focused day, stored date-only (`yyyy-MM-dd`) via
- *   `parseAsIsoDate`. The calendar grid only reads the anchor's date fields, so a
- *   date-only param round-trips losslessly. It is intentionally **nullable** (no
+ * - `anchor` is the focused day, stored date-only (`yyyy-MM-dd`) via the
+ *   local-time {@link parseAsLocalDate} so it matches the calendar's local-time
+ *   date math (no timezone day-shift). It is intentionally **nullable** (no
  *   `.withDefault`): the default anchor is "today", which is dynamic and resolved
  *   per-timezone in the hook (`anchor = param ?? zonedClockDate(now, tz)`). A
  *   clean URL therefore means "today", and navigating back to today clears the
  *   param.
  */
 export const calendarParsers = {
   scope: parseAsStringLiteral(CALENDAR_SCOPES).withDefault(DEFAULT_CALENDAR_SCOPE),
-  anchor: parseAsIsoDate,
+  anchor: parseAsLocalDate,
 } as const
 
 /** Calendar view-state: clean URLs, no back-stack churn. */