How can I copy a date field from Event to opportunity.

Ex: OppABC has a Event with Start & End Date. Event has been created using SF default New Event option

I wanted to copy Start Date of the event if Subject is AAA 

I want to copy VALUE from a field A (Forumula Field) (total number) to FIELD B
FIELD A is a (Data TYe: Formula field) in custom object
FIELD B is a Custom text field in Standard object opportunity.

I want pick list field values from one custom object to copy to Account object Custom pick list field. Is there a easier way to do.
Relation between custom object and Account is by a Look up Field.

I want pick list field values from one custom object to copy to Account object Custom pick list field. Is there a easier way to do.
Relation between custom object and Account is by a Look up Field.