How to calculate days difference between two Dates variable & display the result in the text field?

I am trying to calculate difference between two dates and display the days difference in text field using Bonita BPM Community 7 Version 7.3.1. I am new to Bonita give me some idea that on which event i have to calculate Date difference and display in text field. There is no onselect like events for input type Date in Bonita?

See: http://community.bonitasoft.com/node/24641#node-24646

Hope this helps

regards
Seán

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