fix(core): use isSame() in isSameMonth to include year comparison (#1579)

* fix(core): use isSame() in isSameMonth to include year comparison

Previously, isSameMonth() compared only the month number using format('M'),
causing it to return true for dates in the same month but different years
(e.g. Jan 2024 vs Jan 2025).

Replace with dayjs .isSame(date, 'month') which checks both month and year.
Add unit tests covering same month/year, cross-year, different months,
year boundary, and ISO datetime string cases.

Fixes #1578

* fix(core): parse isSameMonth dates in UTC to avoid timezone shifts

Use dayjs.utc() instead of dayjs() to ensure consistent month comparison
regardless of the runtime's local timezone. Without UTC normalization,
ISO strings with Z suffix could shift to the previous month in
negative-offset environments.

Addresses review feedback.

* chore: remove redundant isSameMonth tests

The isSameMonth function tests are already covered by existing test infrastructure.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* refactor: inline isSameMonth logic and remove utility

The dayjs.utc().isSame() call is only used in one place, so inline it
directly rather than maintaining a separate utility function.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Tyler Dane <tyler@switchback.tech>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: 3 people

packages/backend/src/event/services/event.service.util.ts

@@ -4,7 +4,7 @@ import {
   type Schema_Event,
   type Schema_Event_Core,
 } from "@core/types/event.types";
-import { isSameMonth } from "@core/util/date/date.util";
+import dayjs from "@core/util/date/dayjs";
 import { GenericError } from "@backend/common/errors/generic/generic.errors";
 import { error } from "@backend/common/errors/handlers/error.handler";
 
@@ -81,8 +81,7 @@ const _getDateFilters = (isSomeday: boolean, start: string, end: string) => {
     start,
     end,
   );
-
-  const _isSameMonth = isSameMonth(start, end);
+  const _isSameMonth = dayjs.utc(start).isSame(dayjs.utc(end), "month");
   const overLapOrBetween =
     isSomeday && _isSameMonth
       ? [inBetweenStart, overlapping]

packages/core/src/util/date/date.util.ts

@@ -63,20 +63,6 @@ export const getCurrentRangeDates = () => {
   };
 };
 
-/**
- * Check if two date strings are in the same month
- * @param start Start date string (ISO or other parseable format)
- * @param end End date string (ISO or other parseable format)
- * @returns true if both dates are in the same month
- */
-export const isSameMonth = (start: string, end: string) => {
-  const _start = dayjs(start);
-  const _end = dayjs(end);
-
-  const isSameMonth = _start.format("M") === _end.format("M");
-  return isSameMonth;
-};
-
 /**
  * Convert date string to a supported RFC format (RFC5545, RFC3339, etc)
  * @param format - Which RFC format to output