Updating column (migration)

I have a column in a postgres database that keeps track of a date, but
for some reason I set it up to use a timestamp, ie. my migration says
t.datetime :start_date but I want to switch this to the date type for
my existing table. How can I do this using migrations?

See http://guides.rubyonrails.org/migrations.html, particularly the change_column command

Simon

thank you this was the trick