JSFiddle - React, Tailwind, and code Playground

JavaScript

getHalfHourCount(1100, 1530);
getHalfHourCount(1059, 1531);

function getHalfHourCount(start, end) {
    start = getMinutes(start);
    end = getMinutes(end);
    var diff;
    if (end < start)
        diff = 24*60 - start + end
    else 
        diff = end - start;
    
    if (start % 30 != 0)
        diff = diff + start % 30;
    
    --diff;

    var result = Math.floor(diff / 30);

    document.write(result + "<br />");
    return result;
}

function getMinutes(time) {
    time = String(time);
    if (time.length == 4)
        return parseInt(time.substr(0, 2)) * 60 + parseInt(time.substr(2, 2));
    else
        return parseInt(time.substr(0, 1)) * 60 + parseInt(time.substr(1, 2));
}