Setting date range picker start date to blank

I have a daterange picker based on http://www.daterangepicker.com 's daterange picker and I want to set the start date to empty. This will work for what I want to do.

I'm using an text input field to get the dates

</span><input type="text" class="form-control" id="reservation" />

On the site there is an example of setting "Input Initially Empty" but I couldn't get it to work. Basically I don't know where to set it as it seems.

My daterange picker is inside a partial view and it called by another view. Page scripts are set in the view which calls the partial one. On the daterangepicker.js script I found these lines;

//default settings for options
        this.parentEl = 'body';
        this.element = $(element);
        this.startDate = moment().startOf('day');
        this.endDate = moment().endOf('day');
        this.minDate = false;
        this.maxDate = false;
        this.dateLimit = false;
        this.autoApply = false;
        this.singleDatePicker = false;
        this.showDropdowns = false;
        this.showWeekNumbers = false;
        this.timePicker = false;
        this.timePicker24Hour = false;
        this.timePickerIncrement = 1;
        this.timePickerSeconds = false;
        this.linkedCalendars = true;
        this.autoUpdateInput = true;
        this.ranges = {};

As far as I can tell they are based on moment.js. I tried manipulating this.startDate but couldn't manage to set it to a blank value. using this.startdate = null made the whole date range picker stop working so I guess I need something like empty date equivalent of moment.js. Or something entirely different.

Can anyone show me how to do it?

autoUpdateInput: false

function (chosen_date) {
    $('.form-datetime').val(chosen_date.format('YYYY-MM-DD'));
}