Function addMonth

addMonth(date?: Date, amount?: number): Date | undefined
Returns the month calculated date with the source date and the specified amount

If the target month does not have the same day, the result is clamped to the last day of the target month
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
Author
David Hsing
Example
```
addMonth(new Date(2023, 8, 30), 1);    // Date(2023, 9, 30)
addMonth(new Date(2023, 0, 31), 1);    // Date(2023, 1, 28)
addMonth(new Date(2023, 2, 31), -1);    // Date(2023, 1, 28)
```