Both Explorer and WMP (windows media player) use a CODEC to find the length as well as decode the video. Both Explorer and WMP and incapable of finding these CODECs so you either find the one you need or encode in some way that your current setup can handle.
Frankly I rarely install a CODEC because too many CODECs have been compromised with malware. So I use VLC PLAYER instead.
I have been using Panasonic HD Writer successfully for a year. I am creating a project that is 110 minutes long. HD Writer splits this into 4 MP4 files. But when I try to play the first MP4 it does not play and "Windows Media Player encountered a problem while playing the file" appears. Also, Windows Explorer lists the duration for all MP4 files in my project folder, but not for this MP4 file. If I delete all the MP4 files then do File Output again from HD Writer, it doesn't create the first MP4 file and starts with the second so the first 36 minutes of the project is missing. Something else strange: HD Writer has created CONT, PMPD and TMB files for all my other projects but not this one. Anyone got any suggestions?