.getutc( ) date if end of the month

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Javascript

I have 2 dates : Start date and end date and cash flows based on those dates depending on user input.

If a person doesn't want to enter start date, it will take the end date and print the cash flows.
Example: End Day = 5th of the month, then start date is always (end date + 1 day) i.e. 6th of the month.

Right now my code is working however, if it's the end of the month or 31st, then it's printing 32nd for start date which is incorrect. It should be the 1st since it's the new month. How do i use that condition inside my code? Any help will be greatly appreciated. Thank you.

What I have tried:

if (startDate == null) {
  sDay = endDate.getUTCDate()+1;
}