Because it uses sunset times to change your colour temperature around dusk.  Localtime is not sufficient to work out dusk with any degree of accuracy.<p>Those of us using Redshift are not doing so for a one off colour temperature set, but because it is continually updated through the day and night.<p>You can tell it your location manually or have it done automatically.<p>Redshift is open source.  It does not have binary blobs, patents, a business model and similar crud that f.lux does.  Adjust your paranoia&#x2F;expectations as needed.