MS SQL Query criteria - need to extract date from datetime

I can't for the life of me get this to work. I've tried just about every example I've seen on here and all over the internet. I'm trying to query based on a month's worth of data. The data in the view I'm querying is in this format: 2012-03-20 00:00:00.000. Using the following criteria it's bringing back all of the dates (or nearly all of them). Any ideas?

  AND cast(convert(varchar(10),lag.postingdate,112) as datetime) between '2015-01-01' and '2015-01-31'






Original Query:
     SELECT prov.pgrp_id AS PERFORMING_PROV_ID
     , prov.pgrp_prov_name AS PERFORMING_PROV_NAME
     , lag.chgno AS CHARGE_NUM
     , lag/countcharges AS LAGTIME
     , lag.chgamt
     , lag.postingdate
     FROM dbo.Providers prov
     RIGHT JOIN dbo.LagTime_Charges_Calcs lag ON prov.pgrp_prov_cd =       lag.provcode 
AND prov.pgrp_practice = lag.px 
LEFT JOIN dbo.PlaceofService_union_v pos ON lag.px = pos.px 
AND lag.poscode = pos.poscode
WHERE pos.posid='1' Or pos.posid ='2' AND prov.Laginclude ='y'
and MONTH(lag.postingdate) = 1
and   YEAR(lag.postingdate) = 2015 

--and lag.postingdate between '2015-01-01 00:00:00.000' and '2015-01-31 23:59:59.000'
--AND cast(convert(varchar(10),lag.postingdate,112) as datetime) between '2015-      01-01' and '2015-01-31'