A javascript method using momentjs to get determine the current financial year.

getFy.js

let fy = getFinancialYear();
$('#displayFy').text('Financial year is: ' + fy);
$( "input#datePicker" ).change(function() {
  fy = getFinancialYear($(this).val());
  $('#displayFy').text('Financial year is: ' + fy);
});

function getFinancialYear(date) {
  let financialYear;
  let today = moment();
  if(date){
    today = moment(date, 'YYYY-MM-DD');
  }
  if(today.month() >= 3){
    financialYear = today.format('YYYY') + '-' + today.add(1, 'years').format('YYYY')
  }
  else{
    financialYear = today.subtract(1, 'years').format('YYYY') + '-' + today.add(1, 'years').format('YYYY')
  }
  return financialYear
}