how can i autopopulate todays date into a field when it transitions from 1 state to another

[t]

how can i auto populate today's date into a field when it transitions from 1 state to another?

[GabrielleJ]

In the transition, put a post-function to "The value of field Date of the current issue will be set to the result of a Groovy expression (unless the field already has a value)" where your groovy is new Date().