Sorting problem using AW2.5

I have grid with some of columns as like second student name(string),reg no(integer),Leaving datetime(datetime)...This 2 columns are optional, so it may or may not have values. when i perform a sorting in this two columns it not be a proper one.

when i sort reg no(integer) ascending it showing in the below order
(All the 0 values)
(All the null/blank values)
(All the non-zero, non-null values)

When it sorts the column ascendingly, it should show like below:
(All the null/blank values)
(All the 0 values)
(All the non-zero, non-null values)

then, when i sort the Leaving datetime(datetime) in ascending it showing in the below order
(Some values of non-zero, non-null values)
(All the null/blank values)
(Some values of non-zero, non-null values)

then, when i sort the second student name(string) in ascending it showing in the below order
(Some values of non-zero, non-null values)
(All the null/blank values)
(Some values of non-zero, non-null values)

And also some date columns are working fine in date sorting but not in time,time is not properly sorted. i am using the below code for sorting

var string = new AW.Formats.String;
var number = new AW.Formats.Number;
var date = new AW.Formats.Date; 
date.setDataFormat("ISO8601");
date.setTextFormat("mm-dd-yyyy");

Alex, please help me to out this problem

If you have null value in some columns you should use setErrorValue/setErrorText for proper sorting and display -

http://www.activewidgets.com/aw.system.format/seterrorvalue.html