假设文本中显示的唯一数字是您可以使用的时间：

var txt = 'Class starts at 0845, please be there by 1630 and sign in by 1645.'

getFormattedTime = function (fourDigitTime) {
    var hours24 = parseInt(fourDigitTime.substring(0, 2),10);
    var hours = ((hours24 + 11) % 12) + 1;
    var amPm = hours24 > 11 ? 'pm' : 'am';
    var minutes = fourDigitTime.substring(2);

    return hours + ':' + minutes + amPm;
};
/* replace numeric entities*/
var newTxt = txt.replace(/(\d+)/g, function (match) {
    return getFormattedTime(match)
})
$('body').html(newTxt);

DEMO：http://jsfiddle.net/q6HC9/1
编辑：将时间 Package 在标签中将大大简化情况。使用一个公共类将所有军事时间 Package 在一个span中，然后使用html()方法

<span class="mil_time">0845</span>

getFormattedTime = function (fourDigitTime) {
    /* make sure add radix*/
    var hours24 = parseInt(fourDigitTime.substring(0, 2),10);
    var hours = ((hours24 + 11) % 12) + 1;
    var amPm = hours24 > 11 ? 'pm' : 'am';
    var minutes = fourDigitTime.substring(2);

    return hours + ':' + minutes + amPm;
};
/* find all spans and replace their content*/
$('span.mil_time').html(function( i, oldHtml){
   return getFormattedTime(oldHtml);
})

使用这个：

var getFormattedTime = function (fourDigitTime){
    var hours24 = parseInt(fourDigitTime.substring(0,2), 10);
    var hours = ((hours24 + 11) % 12) + 1;
    var amPm = hours24 > 11 ? 'pm' : 'am';
    var minutes = fourDigitTime.substring(2);

    return hours + ':' + minutes + amPm;
};

s = "Class starts at 1700, please be there by 1630 and sign in by 1645.";

c = s.replace(/([^\d]*)([0-9]{4})([^\d]*)/g, function(match, p1, p2, p3) {
    return p1 + getFormattedTime(p2) + p3
});

console.log(c);

输出：

Class starts at 5:00pm, please be there by 4:30pm and sign in by 4:45pm.

更新

在您的案例中：

s = $("body").html();

c = s.replace(/([^\d]*)([0-9]{4})([^\d]*)/g, function(match, p1, p2, p3) {
    return p1 + getFormattedTime(p2) + p3
});

$("body").html(c);

更新2

如果你在<td class="fourDigitTime">1500</td>中有时间，那么使用这个：

$(".fourDigitTime").each(function() {
    $(this).text(getFormattedTime($(this).text());
});

Live Demo

你可以在正则表达式中使用word boundaries来匹配4位数，然后使用getFormattedTime函数作为.replace的替换函数：

$('body').html(function(_, old) {
    return old.replace(/\b\d{4}\b/g, getFormattedTime);
});

请注意@Doorknob和@Pointy的评论。要仅替换时间“数字”，您需要在语义上标记它们，例如使用html5 <time>标签：

Class starts at <time>1700</time>, please be there by <time>1630</time> and sign in by <time>1645</time>.

$('time').text(function(_, old) {
    return getFormattedTime(old);
});