I am asking end user to put numerical age of the user. For example user inputs 31.

Behind the scene I convert this to the year and store it. The logic is below to convert:

 (moment().subtract(age, 'years')).year();

Which returns the value 1993

Good so far. However, in the other part of the UI where i show this age as number it shows 30 rather 31. The code that converts it back is like below:

 let birthYear =  this.userData.profile.birthYear
     let currYear = moment().year()
     let currDate = moment([currYear, 1, 1])
     let birthDate = moment([birthYear, 1, 1])
     var diffDuration = moment.duration(currDate.diff(birthDate));
     age = diffDuration.years()

I am lost why the age year becomes 30 rather 31.