How can I re-use an expression in a SELECT?

Greetings!

It seems to me, from my uneducated perspective, that if I have a calculated result with a name in a SELECT query, then I ought to be able to re-use the name later in the statement:

select charge,

                case when c.rev_heat_time = 0 then c.pred_heat_time

                else c.rev_heat_time

                end as heat_time,

                case when c.rev_cool_time = 0 then c.pred_cool_time

                else c.rev_cool_time

                end as cool_time,

                fire_date + interval '1 minute' * heat_time as end_of_fire

from charge c

Of course, that fails because heat_time cannot be used to calculate end_of_fire.  I would have to repeat the case statement that defines it instead.  How can I avoid repeating complicated pieces of SELECT queries like this?

Thanks very much!

RobR