Function getStartOfMonth

getStartOfMonth(date?: Date): undefined | Date
Returns the start month date of the given date
Parameters
- Optionaldate: Date
  the date to inspect
Returns undefined | Date
the start month date of the given date
Author
David Hsing
Example
```
getStartOfMonth(new Date(2023, 8, 30));    // Date(2023, 8, 1, 0, 0, 0)
```