InfoPath Dev

Having issues with the code below as I am trying to identify and SAVE the last user who modified the form.  It would work, however when ever I open to VIEW the form, it will show current date and time; however, it is not saved. So make a long story short, I need to it to not populate when viewed and only update when it's saved.

 concat("Last Modified By : ", substring-after(DisplayName, " "), " ", substring-before(DisplayName, ","), " on ", substring(now(), 6, 2), "/", substring(now(), 9, 2), "/", substring(now(), 1, 4), " ", substring(now(), 12, 2) mod 12 + 12 * ((substring(now(), 12, 2) mod 12) = 0), ":", substring(now(), 15, 2), " ", substring("AMPM", 1 + 2 * (substring(now(), 12, 2) >= 12), 2))

Assuming that you formula works, then you only need to use it in an Action, Set a Fields Value Rule prior to saving your form, only set it when a user saves it will be there forever until the next save.

I can only think that you are attempting to use it a s default value?