startOfMonth
import { startOfMonth } from "https://esm.sh/v135/date-fns@3.6.0/index.d.mts";
@example
// The start of a month for 2 September 2014 11:55:00:
const result = startOfMonth(new Date(2014, 8, 2, 11, 55, 0))
//=> Mon Sep 01 2014 00:00:00
function startOfMonth<DateType extends Date>(date: DateType | number | string): DateType;