Execute function in template

I have the following code in my app:

{{#each this.currentItemData['timeline']}}
  {{#each lessons}}
    <span>{{startTime}} - {{endTime}}</span>

startTime and endTime are values from array of objects in data of my page, like that

timeIntervals: [
    id: 1,
    value: '0900',
    label: '09 AM'
    id: 1,
    value: '1000',
    label: '10 AM'

I don’t want to display value field in template, it should be label field, so I wrote a simple function in methods of page:

convertTime : function(value) {
  return this.timeIntervals.find(el => el.value == value).label;

But I can’t call it from template like that:

<span>{{@root.convertTime(startTime)}} - {{@root.convertTime(endTime)}}</span>

However it works if I place it on top on the context and call it like {{this.convertTime(this.constTime)}}

where constTime is some variable from data, just for example.

I’ve tried some different approaches like

{{js 'this.convertTime("0900")'}}
{{js '@root.convertTime("0900")'}}
{{js '@root.convertTime(this.startTime)'}}
{{js '../../convertTime(this.startTime)'}}

but all of them are throwing different errors. Help please!!