Set Display to a Business Year in Scheduler

Yes, this is possible. You need to modify the row total calculation logic so that it only includes the selected events.

The Annual leave tutorial (https://code.daypilot.org/97780/php-annual-leave-scheduling-javascript-html5-frontend-mysql) calculates the row total from all events in a row:

onBeforeRowHeaderRender: function (args) {
  args.row.columns[0].html = "";
  var totalDuration = args.row.events.totalDuration();
  if (totalDuration.days() > 0) {
    args.row.columns[0].html = args.row.events.totalDuration().totalDays() + " days";
  }
},

You'll need to calculate the total manually for events from the selected range:

onBeforeRowHeaderRender: function (args) {
  args.row.columns[0].html = "";
  var totalDuration = args.row.events.forRange("2020-04-01", "2021-04-01")
    .map(function(e) { return e.duration(); })
    .reduce(function(previous, current) { return new DayPilot.Duration(previous.ticks + current.ticks); }, DayPilot.Duration.seconds(0));

  if (totalDuration.days() > 0) {
    args.row.columns[0].html = totalDuration.totalDays() + " days";
  }
},

You may want to further adjust the logic according to your needs.

Thank you.

I also calculated that if I use a start date I require, work out days in year and set that, and add Previous and Next buttons to change the year the numbers work as expected. Would this be correct? (Based on the API returning from StartDate and EndDate)

Just an alternative method.