Stored Procedure Question

I wanted to create a store procedure (unless there is a easier way) so
that i get a person's next birthday date given their birthday.

Aka, in one of my queries i am trying to do:

psql> update users set next_birthday=get_next_birthday(dob) where
user_id='42';

except, i don't quite know how to go about creating the
get_next_birthday function.
'dob' is a field in users that is of type 'date'.

I have tried the following method:

psql> update users set next_birthday=dob + (date_part('year', timestamp
'now') - date_part('year', dob))*365.25;

Which works wonderfully, except when when i if the date of birth is say
'1973-01-10' - in this case, it will put
the next_birthday as 2003-01-10 as it always converts the year to the
current year, not realising that the date/month
have already passed for this year.

Any help would be appreciated,

kind regards,

simran.