Moment.js: First and Last Day of Current Month

Time Zone Aware

Moment.js provides intuitive handling of time zones. Specify UTC for universal time:

let firstDayUtc = moment.utc().startOf('month').format('YYYY-MM-DD'); // Samurai sword cut, we're at Universal Time Coordinated, baby!
let lastDayUtc = moment.utc().endOf('month').format('YYYY-MM-DD'); // At the sound of the beep, the month will be over in UTC... Beep!
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The .utc() converts local time to UTC, thereby circumventing timezone-related issues.

Back to Basics: Native JavaScript

In case Moment.js isn't feasible or feels like an overkill, we have good ol' JavaScript to the rescue:

let date = new Date(), y = date.getFullYear(), m = date.getMonth();
let firstDay = new Date(y, m, 1).toISOString().slice(0, 10); // JS going commando, it's bare-knuckle boxing time!
let lastDay = new Date(y, m + 1, 0).toISOString().slice(0, 10); // JS sends its regards with the last day of the month.
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This vanilla JavaScript rendition crafts exact Date objects, which are then sliced to YYYY-MM-DD.

Fine-Tuned Precision and Formatting

Moment.js shines with its date formatting. Enhance the day with explicit time:

let firstDayWithTime = moment().startOf('month').format('YYYY-MM-DD HH:mm:ss'); // Time to start the stopwatch, the month is Go!
let lastDayWithTime = moment().endOf('month').format('YYYY-MM-DD HH:mm:ss'); // That's it folks! The month is a wrap.
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Appending HH:mm:ss displays time for those requiring time precision.

Date Range Selection with daterangepicker

Weird and complex date requirements? No worries, daterangepicker is your super tool:

let dateRangePickerOptions = {
    startDate: moment().startOf('month'),
    endDate: moment().endOf('month')
};

$('input').daterangepicker(dateRangePickerOptions, function(start, end, label) {
    alert('The gods have bestowed a new range upon us: ' + start.format('YYYY-MM-DD') + ' to ' + end.format('YYYY-MM-DD'));
});
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daterangepicker sets the start and end of the current month as the initial date range, and allows tremendous customizability for UIs.

Date Manipulation Demystified

Moment.js features superb date manipulation methods such as add() and subtract(). Let's fetch the first and last days of the last week:

let lastWeekFirstDay = moment().subtract(1, 'weeks').startOf('week').format('YYYY-MM-DD'); // Let's slingshot back to last week!
let lastWeekLastDay = moment().subtract(1, 'weeks').endOf('week').format('YYYY-MM-DD'); // Alright, last week wound up. Next!
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Future-Proof Your Code

While moment.js is awesome, we need to be aware of deprecated features. Consult the official documentation. Consider smaller alternatives like date-fns or Day.js when tiny bundle size matters.