This is exactly the problem with looping timetables though: real life timetables don't work like this. Almost always, rolling stock diagrams stretch multiple days. Plus, a midnight start is quite inconvenient (for German timetables at least), since the railway's "operational day" tends to start and end at around 0300 hours.
I agree, I was able to let it start at midnight because I had only one passenger service that was active at midnight, I was able to split it up so it starts at Langenprozelten at midnight and ends at Langenprozelten at 23:59:45, but this is not ideal. Also, the portals enable me to let the correct formation "magically" spawn in at a convenient time so it finishes at its start location, as opposed to RL operations. I agree that this is not ideal, but if you want the timetable looping,these are the boundaries that you have to work with until DTG decides that multi-day support is worth developing.
Just remember if go into a second day services start 24:00+ times must be entered, I had freight on my timetable not arriving until 2am , so I made my first timetable 36hrs for overlap allowance
