declare @StartDate datetime,
           @EndDate datetime;
select
    @StartDate = dateadd(hh, 1,dateadd(dd, datediff(dd, 0, getdate()), 0)),
    @EndDate = dateadd(dd, 1, @StartDate);

select ...
from mytable mt
where mt.someeventdatetime >= @StartDate and mt.someeventdatetime < @EndDate;

select ...
from mytable mt
inner join (
	select dateadd(hh, 1, dateadd(dd, datediff(dd, 0, getdate()), 0)) StartDate 
	) DT ON mt.someeventdatetime >= DT.StartDate and mt.someeventdatetime < dateadd(dd, 1, DT.StartDate);

DATEADD(dd, DATEDIFF(dd, 0, DATEADD(hh, -1, someeventdatetime )), 0) RegistryDate

select DATEADD(dd, DATEDIFF(dd, 0, DATEADD(hh, -1, someeventdatetime )), 0) RegistryDate
	, *
from mytable mt