Since you do not seem to have any C++ experience, I recommend not to write any Qt daemon for this task launched by systemd.
I suggest to follow this thread to invoke your application at the date and time desired:
How to add automated tweets via a cron job to WordPress
It is writing about tweets, but I would imagine the concept to be similar. After all, as far as I am aware Jolla intends to remain as much vanilla as possible based on the Mer stack.
Unfortunately, Sailfish OS is currently using an outdated systemd, so you cannot yet use systemd-cron since that requires at least sytemd 197. Although, as far as I know they are working on an update. That will bring the nicer future for your use case.
As indicated in the comments, if you do not have C++ experience, this is probably the easiest route. There will be no direct QML bindings for QProcess, etc, as you would need it.