JSFiddle - React, Tailwind, and code Playground

JavaScript

function GetDay(rawValue) {
    var parts = rawValue.split("-");
    if (parts.length === 3) {
        var day = parseInt(parts[0], 10);
        if (!isNaN(day))
            return day;
    }
    alert("invalid date format");
    return null;
}

alert(GetDay("3-08-2013"));