After Upgrade from 8.2.6 to 8.3.6: function to_timestamp does not exist

Dear list members

I have upgraded my PostgreSQL 8.2.6 to 8.3.6 and I am having trouble
with calling "to_timestamp" function.

Here is the query I use:

SELECT a.*
FROM tblevent a
WHERE to_timestamp(a.from_datetime,'YYYY/MM/DD') >= to_timestamp(
NOW(),'YYYY/MM/DD' )
ORDER BY a.from_datetime

In PostgreSQL 8.2.6 everything works fine.
In PostgreSQL 8.3.6 I get the following error:
------------------------------------------------------------------------
ERROR: function to_timestamp(timestamp without time zone, unknown) does
not exist
LINE 3: WHERE to_timestamp(a.from_datetime,'YYYY/MM/DD') >= to_tim...
^
HINT: No function matches the given name and argument types. You might
need to add explicit type casts.
------------------------------------------------------------------------

Is the "to_timestamp" function not supported anymore in 8.3.6? I could
not read anything about it in the 8.3 documentation.

Regards
Nico