Issue regarding epoch in SQLite

I've done a web application using PHP and postgres. Now, that same application I'm translating to JavaScript and SQLite. I must say, it's not been too tough and SQLite has successfully been able to interpret the same queries as I use in postgres.

Except for this one.

SELECT SUM(t.subtotal)/MAX(EXTRACT(epoch FROM (r.fecha_out - r.fecha_in))/86400) AS subtotal,
COUNT(t.id) AS habitaciones FROM reserva_habitacion t 
LEFT JOIN reserva r ON t.id_reserva=r.id 
WHERE (r.fecha_in <= "2015-03-27" AND r.fecha_out > "2015-03-27") AND r.estado <> 5

Using the FireFox plugin "SQLiteManager" it hints me that the error is this part epoch FROM, but I cannot get my head around it. What am I doing wrong and how could I fix it?

Any suggestions are welcome!