How to set arg value to output of command

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In a launch file it is possible to set a parameter value to the output of a command, e.g. like this:

<param name="date" command="date +'%Y-%m-%d_%I:%M:%S'" />

Is it possible to do the same with an arg?

Background: I would like to have a common directory for all my nodes to store different files in. Therefore I need to pass the directory name to all nodes, which only seems possible via param. With param however, I am not able to make the directory name dependent on the current date for example.

Thank you very much!