How to create an ObjectId from a Date value in the mongo shell?

Question

I want to create a new ObjectId in the mongo shell but for a Date in the past in order to simulate the creation ob this document in the past. That would be the opposite of the getTimestamp() function of an ObjectId (i.e. give a timestamp, get an ObjectId that returns that timestamp when calling getTimestamp on it)

Any ideas how to do this?

robertklep · Accepted Answer · 2013-07-14 18:29:39Z

6

The Mongo shell doesn't seem to support this explicitly. But apart from some timezone stuff, this works:

var timestamp = Math.floor(new Date(1974, 6, 25).getTime() / 1000);
var hex       = ('00000000' + timestamp.toString(16)).substr(-8); // zero padding
var objectId  = new ObjectId(hex + new ObjectId().str.substring(8));

answered Jul 14, 2013 at 18:29

robertklep

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1 Comment

odedfos Over a year ago

Note that for Date you should use a string. The behavior of new Date(1974, 6, 25) is somewhat unexpected. You should use new Date('1974-06-25')

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