One funny "by design" catch.

I want to set variable when channel is answered. execute_on_answer is perfect tool for this job. I have:

<action application="set" data="execute_on_answer=set myvariable=${strepoch()}" />

Everything is working, except that ${strepoch()} is evaluated during dialplan parsing, not when on answer command is executed. So I get time when diaplan is processed, not when user pickups. 

Is there a notation to tell FS not to evaluate variable during dialplan parsing?

Best regards, Gregor