Convert column to date and compare it

[Antonio Tancredi]

I need to compare a text column, which contains a string that represent a date in the format dd/mm/yyyy, to a date.

This is the range (let's suppose that is in another sheet that I cannot change, so I cannot format the column to a date, it must remain a string):

I need to compare C to a date, something like this:

query(importrange(...); "select A, B WHERE C >= '2017-01-08'")

Of course this will not work because only the Aldo row will be shown. Is there any way to convert the column C to a date or format it witt the yyyy-mm-dd pattern?