The <copy> task doesn't not preserve the last modified time on the copied directories when the preservelastmodified attribute it set to true. The copied directories get the current date instead.
Created attachment 28305 [details] Testcase demonstrating the last modified time issue with directories
Created attachment 28306 [details] Patch fixing the last modified date on directories Here a suggestion to fix this issue. It only deals with files and file resources. If a mapper is defined it doesn't attempt to set the last modified time on the directories.