Check fails an input that assert passes

So, the following input:

[
    new Date(2015, 1, 12, 10, 0, 0),
    [new Date(2015, 1, 12, 10, 0, 0), 2],
    new Date(2015, 1, 12, 10, 0, 10),
    [new Date(2015, 1, 12, 10, 1, 0), 2],
]

My code passes it normally in the asserts (it's the very first assert), but when checking it fails on Test 30, which is this same input. I can't figure out why. When I run the code with this input on my PC it gives the right answer too. My code:

function sortFunc(a,b) {

    var A
    var B

    if (a instanceof Array) {A = a[0]}
    else {A = a}

    if (b instanceof Array) {B = b[0]}
    else {B = b}

    return A > B ? 1 : -1
}


function sumLight(els, startWatching?, endWatching?) {

    var bulbs = {1:0, 2:0, 3:0}
    var out = 0
    var bulb = 0

    els.sort(sortFunc)    

    for (let el of els) {       

        if (el instanceof Array) {
            var date = el[0]    
            bulb = el[1]
        } else {
            var date = el
            bulb = 1
        }

        if (!(bulbs[1] || bulbs[2] || bulbs[3])) {
            var start_time = date  
        }

        bulbs[bulb] = 1 - bulbs[bulb]

        if (!(bulbs[1] || bulbs[2] || bulbs[3])) {
            var end_time = date

            if (typeof startWatching != 'undefined') {
                if (typeof endWatching != 'undefined') {
                    out += Math.max(0, Math.min(end_time - start_time, end_time - startWatching, 
                                    endWatching - start_time, endWatching - startWatching))
                } else {
                    out += Math.max(0, Math.min(end_time - start_time, end_time - startWatching))
                }
            } else {
                if (typeof endWatching != 'undefined') {
                    out += Math.max(0, Math.min(end_time - start_time, endWatching - start_time))
                } else {
                    out += end_time - start_time
                }
            }  
        }  
    }


    if (bulbs[1] || bulbs[2] || bulbs[3]) {
        if (typeof endWatching != 'undefined') {
            if (typeof startWatching != 'undefined') {
                out += Math.max(0, Math.min(endWatching - start_time, endWatching - startWatching))
            } else {
                out += Math.max(0, endWatching - start_time)
            }
        }
    }
    return out / 1000
}

Edit:

The checker claims that my function returns "null" for this input. So to test this, I replaced the return line by this:

return out == null ? 60 : out / 1000

However, the checker still claims that my function returns "null". Seems to me that something is wrong with the checker.