Date object and ISO date object return different dates

Question

I need to capture a date value, parse it to an ISODate and then query a mongoDB collection that stores an object with a date value. The query checks for equality between the date of an event and matches it against a function that determines if the date will fall on a weekend. Everything works, however if the date falls on a Monday, it will also come through on the results.

Something I have noticed is the date object moves back a day when we use toISOString:

Date Obj

Mon Apr 18 2016 00:00:00 GMT+0100 (BST)

toISOString:

2016-04-17T23:00:00.000Z

Notice it now has the 17th as the date?

This inconsistency exists in the DB as well:

{
    "_id" : ObjectId("56fe91afceb044f551dbffce"),
    "url" : "http://www.timeoutshanghai.com/features/Blog-Food__Drink/35271/Baristas-showcase-latte-art-in-Shanghai.html",
    "title" : "Baristas showcase latte art in Shanghai - Blog - Time Out - Shanghai",
    "selectedDate" : ISODate("2016-04-17T23:00:00Z"), 
    "__v" : 0
}

This should give selectedDate" : ISODate("2016-04-17T23:00:00Z")

However if we log the collection to the console, the date is returned as a Monday:

  { _id: 56fe91c5ceb044f551dbffcf,
    url: 'http://www.timeoutshanghai.com/features/Blog-Food__Drink/35271/Baristas-showcase-latte-art-in-Shanghai.html',
    title: 'Baristas showcase latte art in Shanghai - Blog - Time Out - Shanghai',
    selectedDate: Mon Apr 18 2016 00:00:00 GMT+0100 (BST),
    __v: 0 },

This becomes an issue when I query the DB to only return events on a weekend as the Monday's are slipping through.

The key is BST. Midnight on day X in BST is 23:00 on day X-1 in UTC (GMT). It's just a time zone difference. — Pointy
– Pointy, Commented Apr 1, 2016 at 15:49
It's the same date but displayed with two different timezones. — Shanoor
– Shanoor, Commented Apr 1, 2016 at 15:55

Dmitri Pavlutin · Accepted Answer · 2016-04-01 15:50:58Z

4

You have 2 different timezones:

toISOString() returns the date formatted in UTC+00:00, which is indicated by the Z flag at the end 2016-04-17T23:00:00.000Z.
Your date object Mon Apr 18 2016 00:00:00 GMT+0100 (BST) (probably formatted using toString()) is formatted in you current timezone: GMT+01:00.

That's why you get this difference in 1 hour.
Read more about the iso date format.

answered Apr 1, 2016 at 15:50

Dmitri Pavlutin

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2 Comments

Rhys Edwards Over a year ago

Okay makes sense. The second date is the one I really need, is there a way to offset the UTC timezone?

Dmitri Pavlutin Over a year ago

@RhysEdwards You'll need a helper function, or the best to use moment library.

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