Change only time part of a datetimeoffset in SQL

Question

How can I set only the time portion of a SQL datetimeoffset variable?

For example:

DECLARE @date datetimeoffset = '2014-01-11 11:04:26 +02:00'

I want to calculate two further datetimeoffsets:

@startdate should be '2014-01-11 00:00:00 +02:00'
@enddate should be '2014-01-12 00:00:00 +02:00'

But all the solutions I tried set the offset to zero.

NOTE: I need the original offset in the results since I need to compare them to some datetimeoffset columns in a table. So just casting to a date will not work.

Further note: This would be quite easy in SQL2012; I could use DATETIMEOFFSETFROMPARTS. Unfortunately, I am not able to upgrade to 2012 at the moment. Example:

SET @startdate = SELECT DATETIMEOFFSETFROMPARTS(datepart(yyyy, @date), datepart(mm, @date), datepart(dd, @date), 0, 0, 0, 0, datepart(tz, @date), 0, 7);

Answer 1

DECLARE @date DATETIMEOFFSET = '2014-01-11 11:04:26 +02:00'

SELECT @date InputDate,
CONVERT(DATETIMEOFFSET,(CONVERT(VARCHAR(20),CONVERT(DATETIME,(CONVERT(DATETIME,CONVERT(DATE,@date)))) ) + ' ' + RIGHT(@date,6))) StartDate,
CONVERT(DATETIMEOFFSET,(CONVERT(VARCHAR(20),CONVERT(DATETIME,(CONVERT(DATETIME,CONVERT(DATE,DATEADD(day,1,@date))))) ) + ' ' + RIGHT(@date,6))) EndDate

Answer 2

DECLARE @date DATETIMEOFFSET = '2014-01-11 11:04:26 +02:00'

SELECT TODATETIMEOFFSET(CAST(@date AS date),DATEPART(tz,@date))
SELECT TODATETIMEOFFSET(DATEADD(day,1,CAST(@date AS date)),DATEPART(tz,@date))