Your clock data is in D registers right? The year in one, the hour in another etc. etc. Although, I have not yet needed the clock data. I believe that since your data registers are constantly changing with time "real time that is", you could use the =,>,<,compare etc. instructions to due other actions as a result of the value of your registers. I'm not sure of what actions you want to happen because you have not yet stated them in your post.
What type of plc are you using... A,F,FX?
Tim