DataFormWebPart cannot sort item with Last modified properly

Question

In my DataFormWebPart, I have set sort by Modifed column. The column is display with follow format:

<xsl:value-of select="ddwrt:FormatDate(string(@Modified), 2057, 5)"/>

However, when sorting with descending order. The time at 12:xx is at higher proirity:

24/04/2015 12:44

24/04/2015 12:25

24/04/2015 17:18

24/04/2015 16:55

24/04/2015 15:52

Why is that?