MySQL has an mathemetical function to generate a random number with a value between 0.0 and 1.0 but this won't help you much, i suppose.
You could also try something like this, which works good on small tables:
SELECT * FROM tablename where B_Posted > '$startdate' and B_Posted < '$enddate' ORDER BY RAND() LIMIT 1
This is not a good idea if your posts table has moore than about 300.000 entries.
Since the startdate and enddate changes each month, you need to write a script anyway.
So generating a random number could be done with php also.
Something like this should work:
SELECT MIN(B_Number), MAX(B_Number) where B_Posted > '$startdate' and B_Posted < '$enddate' FROM tablename;
Fetch the result into $a
$id=rand($a[0],$a[1]);
SELECT * FROM tablename WHERE B_Number>='$d' LIMIT 1
