setAdvertisting serviceData data length max?

Trying to advertise last button press epoch time. Whenever I do something like the following:

var buttonEpoch = 0;

function updateAdvertising() {
    data = totpObj.getOTP("");
    NRF.setAdvertising({0xFFFF : [buttonEpoch]});
}

setWatch(function() {
    var epoch = Math.round(new Date().getTime() / 1000.0);
    buttonEpoch=epoch;
    updateAdvertising();
}, BTN, { edge:"rising", repeat:true, debounce:50 });

Seems like the serviceData is only 2 bytes. Is this a known limitation of using advertising for data?

I am able to put large integers into the manufacturing data but i'm using that now and dont have enough room.