How to add new column based on calculation of another column in postgresql?
I would like to add a new column ...
... I am filtering the data before hand based on a where statement

ALTER'ing a table changes the whole table.
There is no possibility for &quot;filtering&quot;; either every single row gets itself a shiny, new column or none of them do.
alter table t1 
add column date_start date ; 

update t1 
set date_start = time_start::date ; 

But, as others have said, why create a whole new column to store data that is readily available within the table already?