in BsonWriter the if statement is missing an else and thus always falls through to the last else which tries to cast the timespan into a long

case BsonType.Long:
if(obj is TimeSpan)
_writer.Write(((TimeSpan)obj).Ticks);
if (obj is DateTime)
Write((DateTime)obj);
else
_writer.Write((long)obj);
return;

should be

case BsonType.Long:
if(obj is TimeSpan)
_writer.Write(((TimeSpan)obj).Ticks);
else if (obj is DateTime)
Write((DateTime)obj);
else
_writer.Write((long)obj);
return;