setting var's in a property file

[Yevgeni Kovelman]

Hello,
 
I have a standard java property file of type key=value included in my
build.  There is a variable that I would like to set in that property
file during the ant build process.  
 
For example:
 
root=//some//directory//{build-tag}
 
I have a variable $now  defined in my build script, during the build I
would like to replace {build-tag} with $now.  Is there anyway to do that
?
 
Thanks,
Yev