@Lucky1 

sample onChange client script

function onChange(control, oldValue, newValue, isLoading) {
    if (isLoading || newValue == '') {
        return;
    }

    var start = g_form.getValue('variableName');
    g_form.hideFieldMsg('variableName'); // give here your variable name
    var minutes = 120; // 120 mins is 2 hours
    var dateGiven = new Date(getDateFromFormat(start, g_user_date_time_format)).getTime() + minutes * 60000; // add 120mins to date and then compare
    var nowTime = new Date().getTime();
    if (dateGiven < nowTime) {
        g_form.showFieldMsg('variableName', 'Please select date/time after 2 hours from now', 'error');
        g_form.clearValue('variableName');
    }

}

If my response helped please mark it correct and close the thread so that it benefits future readers.