Unless I am totally missing something, wouldn't this do what you're looking for??

declare @lessdate = datetime

declare @greaterdate = datetime

set @lessdate = convert(datetime, '03/07/2008', 103)

set @greaterdate = convert(datetime, '03/07/2008', 103) + 1

update ipadmin.GL_TXN set defunct = 'Y'

where transacted_on >= @lessdate

and transacted_on < @greaterdate

and belongs_to_hcare in (100)