月初

Python

py

def get_bom(d):
  return d - datetime.timedelta(d.day - 1)

PHP

php

function getBOM($d) {
    return new DateTime($d->format('Y-m-01'));
}

Bash

bash

function get_bom
{
    BOM=`date +%Y%m01 -d $1`
    echo $BOM
}

bash

get_bom 20150509

Ruby

rb

def get_bom(d)
  return d - d.day + 1
end