timer 1 => { startDate = 17/01/2019 11PM, endDate = 18/01/2019 9AM }
timer 2 => { startDate = 18/01/2019 7AM, endDate = 18/01/2019 1PM }
timer 3 => { startDate = 18/01/2019 12PM, endDate = 18/01/2019 10PM }
timer 4 => { startDate = 18/01/2019 11PM, endDate = 19/01/2019 9AM }

let sortedTimers = timers.sort((a, b) => {
     return a.startDate.isBefore(b.startDate) ? -1 : 1;
});
let groupedByDate: { [date: string]: Timer[] } = _.groupBy(sortedTimers, timer => {
    let start = this.dateFormatter.toDayOfWeekLongDate(Timer.startDate);
    let end = this.dateFormatter.toDayOfWeekLongDate(Timer.endDate);
    return start == end? start:end; // wrong logic
});