Subversion update externals to a date

Question

I'm working on a large, established project under SVN control. Many parts of the code base are being checked out as externals, but are being actively worked on by other people.
I wanted to update my entire working copy, externals and all, so that it reflected the various repositories' HEADs at a specific point in time. My initial attempt was:

svn up -r'{20090324}' 

This updates the current directory to the specified date, but updates all externals to the current date. Updating the externals one at a time works as expected.
I understand that due to the nature of externals, a single update couldn't work with a revision number, but why doesn't it work with a date?
What's the best way to achieve the point-in-time effect that I'm looking for, without having to maintain a script that hard-codes the various externals?

I'm running a Linux system.

Answer 1

This is inefficient, in that it calls svn update more often than (usually) required. Otherwise, it is short an sweet:

Unix:

find . -name .svn -execdir svn update -r {2010-08-30} \; 

Windows:

forfiles /m .svn /s /c "cmd /c svn up -r {2010-08-30}"