我试了两种方法：

$(document).ready(function()
{
    $('input[type=button]').on('click', function(evt){
        if ($(this).val() == 'Close')
          return true;
        var totalHours = 999; // here I am computing a total number of hours but that is irrelevant to my question
        var reportedTotalHours = parseFloat($("#TotalHours").text());
        var difference = Math.abs(totalHours - reportedTotalHours);
        if (difference >= 0.25) // discrepancy must be less than a quarter hour
        {
          alert("Total of hours for each activity must equal " + reportedTotalHours + ".");
          evt.preventDefault();
          return false;
        }
        return true;
    });
});

return false;应该取消按钮单击，evt.preventDefault();也应该取消按钮单击，但这两种操作都不起作用。单击按钮时会发生表单提交的情况，但我希望在小时数差异小于0.25之前阻止表单提交。