Sql select records for a particular date then AM or PM

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SQL

Hi There,
I have hunted around but can't quite find what I'm doing wrong.

Problem:
What I am trying to do is return records for one particular day then further reduce the results to AM or PM.

I get a "Incorrect Syntax error Near '<'"

Any Ideas what I'm doing wrong in my case statement or a better way to do the same thing?

I just want to pass in the selection criteria as parameters when this is run as a stored procedure. The date selection bit works OK just not the AM PM bit.

I'm Using SQL 2008 R2

Thanks in advance
David.

Declare	@P1 datetime  
Declare @P2 varchar(2)

set @P1 = '2016-06-09 00:00:00.000'
Set @P2 = 'AM'

	SELECT     
	PP4_MAILBACKUP_ID,
	RFC822_txt, 
	SENDER_smtp, 
	[SUBJECT], 
	TOa, 
	ZIPCOMPRESSION 

FROM  Senders
Where CAST(Date_Sent as DATE) = CAST (@P1 as DATE)and (ZIPCOMPRESSION = 0)
AND 
CASE  
	When @P2='AM' Then (DATEPART(hh,Date_Sent) < 12)
	When @P2='PM' Then (DATEPART(hh,Date_sent) >= 12)
END

Answer:
Thanks to Dave Kreskowiak's answer for putting me on the right track.
I certainly was going about it the hard way.
I came up with the following code that seems to work.
Thanks for not giving me the answer as SQL and making me actually think about it :)

Still only P1 and P2 will be passed in to the stored procedure.
The others will be local variables.
BTW, P1 Could be any time of the day not always 00:00:00.000 that was just my example time.

Declare	@P1 datetime  
Declare @P2 varchar(2)
Declare @StartTime datetime
Declare @FinishTime datetime
Declare @SearchDate date

set @P1 = '2016-06-09 00:00:00.000'
Set @P2 = 'AM'

Set @SearchDate = Cast(@P1 as DATE)

if(@P2 = 'AM')
Begin
	-- Set Start time to Midnight (at the befining of the day)
	set @StartTime = CONVERT(DateTime, DATEDIFF(Day, 0, @P1))
	-- Set Finish time to Noon
	Set @FinishTime = DATEADD(hh,12,@StartTime)
	
End

if(@P2 = 'PM')
Begin
	-- Set start time to Midnight (at the begining of the day)
	set @StartTime = CONVERT(DateTime, DATEDIFF(Day, 0, @P1))
	-- Set start time to Noon
	set @StartTime = DATEADD(HH,12,@StartTime)
	-- Set Finishtime to Midnight (at the end of the day)	
	Set @FinishTime = DATEADD(HH,12,@StartTime)
End
Print @StartTime
Print @FinishTime

SELECT     
	PP4_MAILBACKUP_ID,
	DATE_sent,
	RFC822_txt, 
	SENDER_smtp, 
	[SUBJECT], 
	TOa, 
	ZIPCOMPRESSION

FROM  Senders
Where CAST(Date_Sent as DATE) = @SearchDate
And (ZIPCOMPRESSION = 0)
And DATE_sent >= @StartTime
And DATE_sent < @FinishTime
Order by DATE_sent

What I have tried:

Various different formats for the Case Statement.