SP converting date time to date and time separately

I have a SP that will compare a datetime field with today's date
I also want to extract the time only for a separate column

I tried the following:

    DECLARE @today [DATETIME] = CONVERT([DATETIME], CONVERT([DATE], GETDATE()));

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The first one will return todays date with zeros in the time part of the field (2017-04-18 00:00:00.000)

In order to compare to my "date_start"  field which contains time, I need to 'remove' the time portion or make it all zeros.
I tried using the

    DECLARE @datetime_start_today [DATETIME] = CONVERT([DATETIME], CONVERT([DATE], datetime_start));

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I get an error saying 'datetimestart'  field does not exist, when in fact it does.

Once that is fixed (I assume my syntax is incorrect). I need to compare both dates

WHERE @datetime_start_today = @today

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The above should compare only the dates and not the times.

Finally I also want to extract the time only so it shows on the screen as "10:00 AM"  for example