select function alias

I've a series of timestamp columns I'd like to alias in select statements. psql indicates my alias doesnt exist after 

I define it. 

Example -> select to_char(impressions_create_date,'yyyy-mm-dd') as ymd from impressionsdb where ymd like '2016-11%' ;

psql always complains column ymd does not exist. I've inherited a series of tables with many timestamps and would like to 

shorten the select in queries without renaming any columns.

postgresql 9.2 on Gentoo 4.4.26

TIA, TRB