From Now() to Current_timestamp in Postgresql

Use an interval instead of an integer:

SELECT *
FROM table
WHERE auth_user.lastactivity > CURRENT_TIMESTAMP - INTERVAL '100 days'

You can also use now() in Postgres. The problem is you can't add/subtract integers from timestamp or timestamptz. You can either do as Mark Byers suggests and subtract an interval, or use the date type which does allow you to add/subtract integers

SELECT now()::date + 100 AS date1, current_date - 100 AS date2

Here is an example ...

select * from tablename where to_char(added_time, 'YYYY-MM-DD')  = to_char( now(), 'YYYY-MM-DD' )

added_time is a column name which I converted to char for match