How to properly set time windows for backups?

Hello everyone.

I’m trying to configure incremental and full backups for our work’s two fileservers.

We’re running the server on Ubuntu 16.04LTS with UrBackup 2.1.19, the first fileserver is a Windows Server 2003 SP2 machine (I know, it’s old) with the latest 1.4.x version of UrBackup and the second fileserver is Windows Server 2008R2 SP1 machine with the client 2.1.15.

We only want file backups, image backups are turned off. The time schedule for backups is the same for both servers and should be following:
Incremental file backup: 1-6/18-24
Full file backup: 7/0-24

The time it takes to make an incremental backup is about 40-50 minutes and full file backup takes about 6 hours on each server.

Now, the problem seems to be this. The full file backup starts only every two weeks, not every week as it should. Also, not every day, but on some days, the incremental backup skips one day.
I read in the admin guide:

The server detects that the time to the last incremental backup is larger than the interval
for incremental backups or the last time to the last full backup is larger than the interval
for full backups.

How exactly does this works? Does the server count the last backup time as the time when the last backup started, or the time when the last backup finished?
Because it seems, that if the backup (incremental, for example) starts at 23:50 Monday, it finishes at 00:30 on Tuesday. And then, because there was a backup that already “happened” on Tuesday, it doesn’t start the next backup on Tuesday evening/night and waits until Wednesday evening/night for the next backup.

Do I understand this behavior correctly? How can I force to start the backup at the beginning of the time window?

Thanks for advice!

Also, in activities menu, I can see that the backup started on lets say 21.05.2017 at 00:15, but when I check logs, I can see that the “Starting scheduled incremental file backup…” shows at 20.05.2017 at 23:09.

After this backup, there wasn’t any other backup done on 21.05.2017 in the evening/night, probably because the previous backup actually already happened on this day…? Am I correct here?

Building on the above, I should set the backups to start earlier, so they can finish that same day even if they start at the very last minute of the backup window, right?