Zoho crm Reminder Set in meeting

Hi team,

I have code for the reminder notifications one hour before but for some of the hours, its is not working to update.

Requirement :

When the record is created, the reminder should automatically be set to 1 hour before the start time.

here is the code :

resp = zoho.crm.getRecordById("Events",eventId);
istDate = resp.get("Start_DateTime");
info "test - "+resp.get("Remind_At");
day = istDate.subString(8,10);
strhour = istDate.subString(11,13);
min = istDate.subString(14,16);
hourSub = strhour.toNumber() - 01;
timeOnehoursBefore = istDate.toString("yyyy-MM-dd'T'" + hourSub + ":" + min+":00+05:30");
EventRecordsUpdate = Map();
EventRecordsUpdate.put("Remind_At",timeOnehoursBefore);
info EventRecordsUpdate;
if(EventRecordsUpdate != null)
{
updateEventRecords = zoho.crm.updateRecord("Events",eventId,EventRecordsUpdate);
}
info eventId;
resp = zoho.crm.getRecordById("Events",eventId);
info resp.get("Remind_At");

Some start and end date time working but some of the records is not working.
Like less than 6:00-7:00 PM working but greater the 7:00-8:00 PM not working.
What will do any idea for this issue? Because timing is set to near to the current time but not working for update start date time to one hour before.

Thanks