Applying a formula randomly throughout a time series

[Ricky2CU]

Is there a way to randomly apply a formula given a timeseries? For example, if I want to apply a formula for only a specific given number of hours for any given month in a random fashion, how would I do that?

i.e. if for March I only want to have this formula work for a random set of 20 hours

Any help or thoughts would be great.

 

[GlennUK]

There's not information on what you are trying to do. Be more specific, and give examples of what you are trying to achieve.