Function addMonth

addMonth(date?, amount?): Date | undefined
Returns the month calculated date with the source date and the specified amount
Parameters
- Optionaldate: Date
  the date to inspect
- Optionalamount: number
  the amount to add, negative number means minus
Returns Date | undefined
the month calculated date with the source date and the specified amount
Example
```
addMonth(new Date(2023, 8, 30), 1);    // Date(2023, 9, 30)
```