momentjs calculates date difference incorrectly - momentjs

Question

In my angular web application, I want to compare two dates to see if a person is less than 18 years old when she/he entered the company. Here is the code I use to do this:
const dayOfBirth = moment(formControl.value, this.dateFormat, true).startOf('day');
const entranceDateControl = this.wizardFormGroup.get('entranceDate');
const entranceDate = moment(entranceDateControl.value, this.dateFormat, true).startOf('day');
// Check validation rule R3: Age is less than 18 compared to entrance date
const difference = moment.duration(Math.abs(entranceDate.diff(dayOfBirth)));
if (difference.years() < 18) {
const validationMessage = this.getValidationMessage('R3', formControlName);
return validationMessage ? validationMessage.message : null;
}
As you can see, I am using startOf('day') to get rid of any time component so that I only handle dates. I use diff() to get the difference between two dates and then duration() to convert the difference to years, months, days, etc. Using this code, the validation message should NOT show when the person is turning 18 years old on the day when she/he entered the company.
Upon testing this, I came across what is, in my opinion, strange behavior. Depending on months and years used, it gave different results. For instance, for these dates it was Ok:
dayOfBirth = 1998-03-01, 1998-04-01, ..., 2000-02-01
entranceDate = 2016-03-01, 2016-04-01, ..., 2018-02-01
But the following dates returned the validation message:
dayOfBirth = 2000-03-01, 2000-04-01, ..., 2002-02-01
entranceDate = 2018-03-01, 2000-04-01, ..., 2020-02-01
After these dates, i.e. using 2002-03-01 and onward, it works again. I also got wrong result for the dates preceding 1998-03-01.
Now, I had a closer look at the Duration object and I noticed that for the times where it was less than 18 years, it had calculated 864 milliseconds less then when it came to the right conclusion that it was 18 years between the dates.
Correct duration
----------------
dayOfBirth = 1998-03-01, 1998-04-01, ..., 2000-02-01
entranceDate = 2016-03-01, 2016-04-01, ..., 2018-02-01
Duration = 568080000000 ms
Wrong duration
--------------
dayOfBirth = 2000-03-01, 2000-04-01, ..., 2002-02-01
entranceDate = 2018-03-01, 2000-04-01, ..., 2020-02-01
Duration = 567993600000 ms
Duration difference
-------------------
568080000000 - 567993600000 = 86400000 ms = 24 hours = 1 day
Has anyone an explanation for this? Can it be considered a bug in momentjs? Any viable workaround for this?

Answer 1

I didn't go into details in moment source code but it seems duration() is playing tricks with you. Simplify the code and rely only on diffas follow and you should be good (at least it seems to work for the samples you provided). And it's easier on the eyes :)
const moment = require('moment')
const dayOfBirth = moment('2000-03-01').startOf('day');
const entranceDate = moment('2018-03-01').startOf('day');
const difference = entranceDate.diff(dayOfBirth, 'years')
if (difference < 18) {
console.log( '<18')
} else {
console.log( '>=18')
}
will output >=18