Time In State

I've got a SQL that registers people passing in and out via their acess cards. Basically passing in means they set a discrete tag to 1 and out tag to 0. SQL logs delta so change is logged. I would now want to Query the database and just want the monthly time they've been in i.e. how long has the tag been 1.
I can pull a monthly list of the tags all statuses (1 and 0) and times but how do I put together a query that just gives me the time it been in 1 status?

following to pull a delta list i.e changes:

SET NOCOUNT ON
DECLARE @StartDate DateTime
DECLARE @EndDate DateTime
SET @StartDate = DateAdd(wk,-1,GetDate())
SET @EndDate = GetDate()
SET NOCOUNT OFF
SELECT TagName, DateTime = convert(nvarchar, DateTime, 113), vValue, Quality, QualityDetail = v_History.QualityDetail, QualityString
FROM v_History
LEFT JOIN QualityMap ON QualityMap.QualityDetail = v_History.QualityDetail
WHERE TagName IN ('Pete_Smith')
AND vValue <= 1
AND wwVersion = 'Original'
AND wwRetrievalMode = 'Delta'
AND wwRowCount = 1000
AND DateTime >= @StartDate
AND DateTime <= @EndDate




heson1