Hello,

I think there is an error in the implementation of $dateDiff aggregator operator. If I write something like this:

```
{
$dateDiff:{startDate:{$toDate:"1976-01-18"}, endDate:"$$NOW",unit:"year"}
}
```

it return 47 and not 46.

The documentation says what I am expecting:

The `$dateDiff`

expression returns the integer difference between the `startDate`

and `endDate`

measured in the specified `units`

. Durations are measured by counting the number of times a unit boundary is passed. For example, two dates that are 18 months apart would return 1 `year`

difference instead of 1.5 `years`

.

Anyone else seeing this?

Thanks in advance

I think it’s correct - think of both dates truncating to January 1st of the same year and then subtract them. I get 47 years, don’t you?

Asya

Thank you Asya! But I cannot find where it says that truncates the dates before subtract. The example listed says only: Difference 18 months → 1 year difference (and not 2). This way I understood that it does the subtract and then get the duration in unit (with floor)

I think the key is the phrase “Durations are measured by counting the number of times a unit boundary is passed.” so 1.5 years is only 1 year if the year boundary (Jan 1) is passed once, if it’s passed twice then it’s 2 years. So from December 28th 2022 to today is one year (because we pass year boundary), same as January 2nd 2022 to today.

Asya

Ok but if it is really implemented this way, I would update the documentation because the example show something else. I would like to use this feature for age calculation but this way is unusable… Thanks anyway