How can i get control of the create issue page in JIRA

[Rahul TataElxsi]

Here is my requirement -

In the create issue page i have created some CUSTOM DATE fields (END-DATE)and a duration field.I want to calculate the end-date custom field based on the start date and duration field provided by user while creating the issue.

Is there any way i can do this via a plugin.I am able to do this in the EDIT of an issue usinga plugin , but dont know how to get control over the CREATE-ISSUE  PAGE and do the calculation via a plugin.

Please help on this.