Age to Birth Date calculator

This package is a simple tool to calculate the earliest or latest possible date of birth for a given age. Of course with typescript support.

npm install age-to-birth-date

import {
    latestBirthDateForAge, 
    earlistBirthDateForAge,
} from 'age-to-birth-date'

// Assuming todays date is 2019-09-10 (00:00z)

earlistBirthDateForAge(20); // Date(1998-09-10)
latestBirthDateForAge(20); // Date(1999-09-10)

// CommonJS
const ageToBirth = require('age-to-birth-date');
console.log(ageToBirth.earlistBirthDateForAge(20)); // Date(1998-09-10)

The example above is inclusive, meaning that if you were born exactly on 1999-09-10, you would still be regarded as being 20 years of age (actually you would be 20 years and 0 days).

Likewise, if you were born at 1998-09-10 you would be regarded as 20, although the actual age is 21 years and 0 days.

This can be prevented by passing false as the second parameter:

// Assuming todays date is 2019-09-10 (00:00z)

earlistBirthDateForAge(20, false); // Date(1998-09-11) (19 years, 11 months, 30 days) 
latestBirthDateForAge(20, false); // Date(1999-09-11) (18 years, 11 months, 30 days)

Basic comparisons

To make sure that some one is at least 10 years old.

// Assuming todays date is 2019-09-10 (00:00z)
const usersBirthDate = new Date('2008-05-10:00:00:00z');

const latestFor10YO = latestBirthDateForAge(10) // 2009-09-10
const foo = usersBirthDate < latestFor10YO; // true

Dates between an age span

Package also includes dateRangeForAgeSpan that returns a object with from and to dates calculated from the input ages. For example:

import { dateRangeForAgeSpan } from 'age-to-birth-date'

// Assuming todays date is 2019-09-10 (00:00z)

const foo = dateRangeForAgeSpan(10, 20);
/**
    foo = {
        from: Date(1998-09-10:00:00:00z)
        to: Date(2009-09-10:00:00:00z)
    }
*/

This method also accepts an optional third boolean parameter to not be include this day:

// Assuming todays date is 2019-09-10 (00:00z)

const foo = dateRangeForAgeSpan(10, 20, false);
/**
    foo = {
        from: Date(1998-09-11:00:00:00z)
        to: Date(2009-09-11:00:00:00z)
    }
*/

Why did I do this?

Working with dates and ages always gets to me - it seems I just can't wrap my head around if I am supposed to subtract or add years and days - and in what direction to get what results. And so on... Every time I end up spending a lot of time just trying to understand. Perhaps it's just me that are slow in this sense...

But if there is some one else out there that hates calculating ages and birth dates - feel free to use this package.