You can convert the seconds (as returned by BASS_ChannelBytes2Seconds) to a TimeSpan instance like this:
TimeSpan ts = TimeSpan.FromSeconds(length);
To convert seconds to a DateTime value you must first create a new DateTime instance with no hour/minute/second portion and then add the seconds:
DateTime dt = DateTime.Today;
You might also want to use the "Utils.FixTimespan" method to convert the seconds to a printable string (see the docs for more info).