Fix schedules invalid dates issue (#3541)

# What this PR does

Fix schedules invalid dates issue

## Which issue(s) this PR fixes

https://github.com/grafana/support-escalations/issues/8084

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Fixed
+
+- Fix schedules invalid dates issue ([#support-escalations/issues/8084](https://github.com/grafana/support-escalations/issues/8084))
+
 ## v1.3.76 (2023-12-11)
 
 ### Fixed

grafana-plugin/src/pages/schedule/Schedule.helpers.ts

@@ -16,10 +16,13 @@ const mondayDayOffset = {
 };
 
 export const getWeekStartString = () => {
-  if (!config.bootData.user.weekStart || config.bootData.user.weekStart === 'browser') {
+  const weekStart = (config.bootData.user.weekStart || '').toLowerCase();
+
+  if (!weekStart || weekStart === 'browser') {
     return 'monday';
   }
-  return config.bootData.user.weekStart;
+
+  return weekStart;
 };
 
 export const getNow = (tz: Timezone) => {