Hi, I am seeking advice for a tested WordPress plugin to create:

 

- One line entry form (a string of few alphanumeric values) which would auto calculate the result (that would be the date) to be displayed adjacent to the entry form.

 

Has to work on smartphones when browsing my site as well as on PC.

 

Plugin to be simple enough to allow me to enter formulas.

 

Plugin has to be free as I intend to create that feature as a free online service.

 

Suggestions please