I would do it like this:
Get a random number for the amount of blogs zenno should post.
Get a random hour/minutes per blog that should be posted. (since zenno doesn't run all day, you should input min. and max. limits)
Use the macros 'get current hour'/ 'get current minutes'
If the current hour/minutes is bigger than one of the random hour/minutes, then zenno should post.
And it will do this for all the needed posts.