If your sql source contains a 'created' field then you can use a where clause with the created field. If your sql source table does not contain a 'created' field then you can't do that, but you will need to use the original unix timestamps from the 'myunix_time' field. What kind of function you need to convert the unix timestamp to a date depends on what database system you're using. SQL Server, Oracle, MySql etc all have different functions for dealing with unix timestamps.
However, the data returned is not limited to those with 'created' after 2015-03-04 21:00:00.
I have some data with 2015-03-03 in the created field.
I think it returns all data from the table.
I also find something funny. The script returns all data from the table when I put created greater than certain timestamp in the where clause. (i.e. WHERE created > unix_timestamp('2015-03-04 21:00:00')
But no data at all return when I put an extra timestamp condition in the where clause like below.
WHERE created > unix_timestamp('2015-03-04 21:00:00')
and created <unix_timestamp('2015-03-05 21:00:00')