How to compare the modifications between a local file and a file from ftp

I'm working on an updater client and I want to compare the last modification dates between a local file and a file from the ftp server. While doing this, I should not consider the time on the local system. However, QFileInfo::lastModification() functions always returns the modification date on the local system. I would appreciate any suggestions please.

QFtp (or better QNetworkAccessManager) -> download the file in a temporary file (QTemporaryFile)
compare local file and temporary file... (size... QCryptographicHash::Sha1 of files...)