DIY Date manipulations in JavaScript: how to get days, weeks, and months from now with chainable utility methods.

toDate.js

/* Define new prototype methods on Date object. */
// Returns Date as a String in YYYY-MM-DD format.
Date.prototype.toISODateString = function () {
  return this.toISOString().substr(0,10);
};

// Returns new Date object offset `n` days from current Date object.
Date.prototype.toDateFromDays = function (n) {
  n = parseInt(n) || 0;
  var newDate = new Date(this.getTime());
  newDate.setDate(this.getDate() + n);
  return newDate;
};

// Returns new Date object from start of week of current Date object
// optionally offset `n` weeks from week of current Date object.
Date.prototype.toStartOfWeek = function (n) {
  var newDate = new Date(this.getTime());
  newDate.setDate(this.getDate() - this.getDay());
  return n ? newDate.toDateFromDays(n * 7) : newDate;
};

// Returns new Date object from start of month of current Date object
// optionally offset `n` months from month of current Date object.
Date.prototype.toStartOfMonth = function (n) {
  n = parseInt(n) || 0;
  var newDate = new Date(this.getTime());
  newDate.setMonth(this.getMonth() + n, 1);
  return newDate;
};

/* Instantiate a Date. */
var today = new Date();

/* Chain all the things. */
console.log(
  'Today:               ', today.toISODateString()
);
console.log(
  '7 days ago:          ', today.toDateFromDays(-7)
                                .toISODateString()
);
console.log(
  '30 days ago:         ', today.toDateFromDays(-30)
                                .toISODateString()
);
console.log(
  '90 days from now:    ', today.toDateFromDays(90)
                                .toISODateString()
);
console.log(
  'Start of this week:  ', today.toStartOfWeek()
                                .toISODateString()
);
console.log(
  'End of this week:    ', today.toStartOfWeek(1)
                                .toDateFromDays(-1)
                                .toISODateString()
);
console.log(
  'Start of last week:  ', today.toStartOfWeek(-1)
                                .toISODateString()
);
console.log(
  'End of last week:    ', today.toStartOfWeek()
                                .toDateFromDays(-1)
                                .toISODateString()
);
console.log(
  'Start of next week:  ', today.toStartOfWeek(1)
                                .toISODateString()
);
console.log(
  'End of next week:    ', today.toStartOfWeek(2)
                                .toDateFromDays(-1)
                                .toISODateString()
);
console.log(
  'Start of this month: ', today.toStartOfMonth()
                                .toISODateString()
);
console.log(
  'End of this month:   ', today.toStartOfMonth(1)
                                .toDateFromDays(-1)
                                .toISODateString()
);
console.log(
  'Start of last month: ', today.toStartOfMonth(-1)
                                .toISODateString()
);
console.log(
  'End of last month:   ', today.toStartOfMonth()
                                .toDateFromDays(-1)
                                .toISODateString()
);
console.log(
  'Start of next month: ', today.toStartOfMonth(1)
                                .toISODateString()
);
console.log(
  'End of next month:   ', today.toStartOfMonth(2)
                                .toDateFromDays(-1)
                                .toISODateString()
);

This micro-library was inspired by: http://stackoverflow.com/questions/37567569/javascript-how-to-get-dates-for-seven-days-ago-last-month-this-week

The following methods can be copied and used independently: .toISODateString, .toDateFromDays, .toStartOfMonth

However, the method .toStartOfWeek is dependent on .toDateFromDays.

All methods except for .toISODateString — which returns a string — return chainable Date objects that can be chained with other chainable native Date methods.