Function isLeapYear

isLeapYear(dateYear?): boolean
Returns whether the date or year is a leap year
Parameters
- OptionaldateYear: number | Date
  the date or year to check
Returns boolean
whether the date or year is a leap year
Example
```
isLeapYear(2000);    // true
isLeapYear(new Date(2000, 0, 1));    // true
```