Nrthugu

So it's pretty straight forwards:

import startOfMonth from 'date-fns/start_of_month';



export const getFirstDayThisMonth = date => {

  const firstDay = startOfMonth(date);

  return firstDay;

};

given input (for example):
1987-02-02T00:00:00.000Z

it returns:
1987-01-31T23:00:00.000Z

The error is reproduced exactly when trying to produce the first date according to the method mentioned in this answer:

var date = new Date();

var firstDay = new Date(date.getFullYear(), date.getMonth(), 1);

The error comes when running jest-tests and can't be reproduced in the console where it works as expected:

const day1 = new Date('1987-02-02');

undefined

const day2 = new Date(day1.getFullYear(),day1.getMonth(),1)

undefined

day2

Sun Feb 01 1987 00:00:00 GMT+0100 (Central European Standard Time)

If in the second solution I also set the fourth argument (hour) to 1, it correctly handles:

1987-02-02T00:00:00.000Z

returning 1987-02-01T00:00:00.000Z

and

2001-03-03T00:00:00.000Z

returning 2001-03-01T00:00:00.000Z

but

2002-04-04T00:00:00.000Z

returns 2002-03-31T23:00:00.000Z

I'm really at loss as to what could be causing this issue, it feels like there's some rollover to the previous date when the time is set to 00:00:00?

Per request, here's the date-fns-code for startOfMonth:

function startOfMonth (dirtyDate) {

  var date = parse(dirtyDate)

  date.setDate(1)

  date.setHours(0, 0, 0, 0)

  return date

}

I really don't get why

var date = new Date();

var firstDay = new Date(date.getFullYear(), date.getMonth(), 1, 1);

seem to work on feb-march but not on april?