Leap Year: isValid bug #2607

[iSanjayAchar]

Javascript (and most other programming languages) recognize that February only has 28 days (or 29 in a leap year). So, when you enter a date like "2024-02-31" using new Date("2024-02-31"), it attempts to create a valid date.

Here's what happens behind the scenes:

• Javascript creates a Date object with the provided year, month (0-indexed, so February is 1), and day.
  It recognizes February doesn't have 31 days.
• To create a valid date, Javascript automatically adjusts the date to the closest valid date. In this case, it becomes March 1st (the 1st day of the next month).
• This behavior is to ensure the Date object represents a real date within the calendar system.

This PR adds an additional check to isValid method ensuring the date matches before checking for validity

Before

  isValid() {
    return !(this.$d.toString() === C.INVALID_DATE_STRING)
  }

After

  isValid() {
    switch (this.$D !== this.$d.getDate()) {
      case true:
        return !(this.$d.toString() === C.INVALID_DATE_STRING)
      default:
        return false
    }
  }