how to set time field in extra fields

Hi.
I would like to set a custom time in an extra field.
I want to be available to choose a time between 8 am and 8 pm, for example.
For minutes combobox I would like to have just hour:15 , hour:30 and hour:45 and hour:00 as well.
Can you help me please?

http://wordpress.org/extend/plugins/si-contact-form/