Aleksandr Chirkov, у свойства "value" нет свойства "innerHTML".
function showCurrentDate()
{
var getCurrentDay = new Date();
var year = getCurrentDay.getFullYear();
var month = getCurrentDay.getMonth();
var day = getCurrentDay.getDate();
var value1 = year + "-" + month + "-" + day ;
document.getElementById("calendarForTasks").value=value1;
}