[jQuery] iterate through form elements, wait for post to finish and continue

[Mark Steudel]

I have the following inputs

    <input type="text" id="start_date" value="2009-01-01" /><br/>
    <input type="text" name="a" id="a" value="3 days" class="interval"
rel="a_preview"/><span id="a_preview"></span><br />
    <input type="text" name="b" id="b" value="3 days" class="interval"
rel="b_preview"/><span id="b_preview"></span><br />
    <input type="text" name="c" id="c" value="3 days" class="interval"
rel="c_preview"/><span id="c_preview"></span><br />

    <input type="button" id="assign" name="assign" />

The idea is that I have a php script that calculates the date by
building upon the values that are passed into it.

So in the (a|b|c)_preview span the expected values would be something
like:
2009-01-04
2009-01-05
2009-01-06

I then tried to do something like the following:

$(document).ready(function(){
            $("#assign").click(function() {
                $(".interval").each( function() {
                    // save current
                     var previewId = $(this).attr("rel");
                      //alert( previewId );
                     if( $("#" + $("#previousStartDate").val()).val() !
= '' && $("#" + $("#previousStartDate").val()).val() != null) {
                         var startDate = $("#" + $
("#previousStartDate").val() ).html();
                     }
                     else {
                         var startDate = $("#start_date").val();
                     }
                     alert( '0-'+previewId);
                     $.post("/scripts/admin.course.php?
action=calculateDate", {startDate: startDate, nextDate: $(this).val
()},function(data){
                         $("#" + previewId).html(data);
                          alert( 'i-'+previewId );
                          // save what the previous start date field
id is
                          $("#previousStartDate").val($(this).attr
("rel") );
                     });
               });
            })
        })

What happens (which from what I read is expected) is that I get all of
the "O-" alerts and then I get all of the "I-" alerts. So the dates
aren't built upon each other.

What would be a good way to re-do this function so that it waits for
the previous value before continuing on.