Immutable Moment - Like Moment.js but without surprises
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Immoment = Immutable Moment

npm install
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It works just like Moment.js but without surprises.

This project is very young and is a work in progress.

The 0.1 version supports some methods on the immoment object itself, but durations are not encapsulated yet. Still it is already useful for simple use cases.

For example, in Moment when you write:

var january1 = moment('2015-01-01');
var february1 = january1.add(1, 'months');
var march1 = january1.add(2, 'months');

you may be surprised that march1 is now set to the April Fools' Day.

Using Immoment after running the above code march1 would be March 1 which some people may find somewhat less surprising.


  • The 0.0.x is the first working experiment
  • The 0.1.x has methods like immoment.min() working
  • The 0.2.x will have durations properly encapsulated
  • ...

Contact me with any suggestions.


The minified version of Immoment is less than 0.5kB and less than 0.25kB gzipped.


The performane of Immoment is around 97% of the performance of Moment for a typical use case. See benchmarks:

If your programs do anything else besides calling methods on Moment objects then you are not likely to see even that 3% hit.


Install to use in your Node project, updating the dependencies in package.json:

npm install immoment --save


To use it in place of moment:

var moment = require('immoment');

Usage in browser

You need to load both moment.js and immoment.js (in that order). Example with CDN:

<script src=""></script>
<script src=""></script>

See the latest version of Moment on:

And the latest version of Immoment on:


var immoment = require('immoment');
var moment = require('moment');

var mJan1 = moment('2015-01-01');
var mFeb1 = mJan1.add(1, 'months');

var iJan1 = immoment('2015-01-01');
var iFeb1 = iJan1.add(1, 'months');

console.log('moment - Jan1 is ' + mJan1.format('MMM D'));
console.log('moment - Feb1 is ' + mFeb1.format('MMM D'));

console.log('immoment - Jan1 is ' + iJan1.format('MMM D'));
console.log('immoment - Feb1 is ' + iFeb1.format('MMM D'));

It outputs:

moment - Jan1 is Feb 1
moment - Feb1 is Feb 1
immoment - Jan1 is Jan 1
immoment - Feb1 is Feb 1


For any bug reports or feature requests please post an issue on GitHub.


Rafał Pocztarski
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This is free software released under the Expat (MIT) License.

It is a permissive, non-copyleft, GPL-compatible license that complies with:

See for details.