helper caching issue

I have a helper:

def pretty_datetime( dt )
  dt.strftime( "%b %d, %Y %I:%m%p" )

and I have a partial that uses the helper like this:

<%= pretty_datetime alert.updated_at %>

The problem is when I update the record the previous updated_at value
is still being displayed.

When I don't use the helper I see the updated_at value change every time.

<%= alert.updated_at %>

When I put the helper call back again, the partial again shows me the
previous updated_at value, even after multiple updates and me taking
it away and putting it back multiple times. Even if I restart the
server I'm still seeing the old updated_at values from when the record
was first created. I see the values changing in the database when I
update the record so I know the problem isn't there.

So why is the output of my helper (or partial) being cached?

This is in development mode, Rails 2.3.2.