The Austrian one has a date and time stamp.
Sure, but it could have been prepared at t minus 24hrs, t minus 36hrs etc.
Shouldn’t it be based on the time these conditions prevail? Why would it matter when it was prepared (seriously curious)?
We clearly have wires crossed 
This chart is prepared 120hrs ahead (t+120)
If you download this chart tomorrow (also for 1200 20/10) it will say t+96 and it will be more accurate.
As discussed here, a lot of free wx sites present these charts but older versions.
I see. We’re discussing the forecast (and differences among providers) not eg two charts that show prevailing conditions. Are there differences among the latter as well? Or do they start resembling each other the closer to the actual date and time they are?
The different charts should converge as you get nearer the present time, sure. I don’t think they converge as much as one might expect though.
