How to add date and time in SQL Server - sql

Question

I have two variables #date of type datetime and #time of type time. I want to add both to get another datetime variable. And I want to perform further calculations on it.
Ex:
Declare #date datetime
Declare #time time
I want something like this
#date = #date + #time (but not concatenation)
SELECT #Startdate = DATEADD(DAY, -1, #date )
Is there any way?

Answer 1

You can tranform your time to seconds and add them to your datetime value:
DECLARE #datetime DATETIME = GETDATE(),
#time TIME = '01:16:24',
#timeinseconds INT
PRINT 'we add ' + CAST(#time AS VARCHAR(8)) + ' to ' + CONVERT(VARCHAR,#datetime,120)+ ':'
SELECT #timeinseconds = DATEPART(SECOND, #time)
+ DATEPART(MINUTE, #time) * 60
+ DATEPART(HOUR, #time) * 3600
SET #datetime = DATEADD(SECOND,#timeinseconds,#datetime)
PRINT 'The result is: ' + CONVERT(VARCHAR,#datetime,120)
Output:
we add 01:16:24 to 2015-07-17 09:58:45:
The result is: 2015-07-17 11:15:09

Answer 2

The only thing you are missing is that #time needs to be cast back to a datetime before adding to #date.
declare #date datetime = '2022-05-26'
declare #time time = '09:52:14'
declare #Startdate datetime
set #date = #date + convert(datetime,#time)
SELECT #Startdate = DATEADD(DAY, -1, #date)
Produces:

Answer 3

If you need to take only date part from #date and time part from #time - can convert your #date and #time to strings, concatenate the values and convert back to datetime:
select cast(convert(nvarchar(20), #date, 104) + ' ' +
convert(nvarchar(20), #time, 108) as datetime2)
Or, alternatively, if you need to add time to datetime value, you can do something like:
select dateadd(ms,
datepart(ms, #time),
dateadd(ss,
datepart(ss, #time),
dateadd(mi,
datepart(mi, #time),
dateadd(hh, datepart(hh, #time), #date))))

Answer 4

First of all convert #date and #time variables to NVARCHAR(), then concat them and after It convert It to DATETIME datatype. After It you can use DATEADD function on It. Try in following:
DECLARE #date DATETIME
DECLARE #time TIME
SET #date = GETDATE()
SET #time = '10:12:13'
SELECT DATEADD(DAY, -1, CAST(CONVERT(NVARCHAR(20), #date, 110) + ' ' +
CONVERT(NVARCHAR(20), #time, 108) AS DATETIME))
OUTPUT (Today day -1 + time '10:12:13'):
2015-07-16 10:12:13.000

Answer 5

I'm not sure what's going on here, but if your variables are datetime and time types, this should work just fine:
declare #date datetime
declare #time time
set #date = '20150717'
set #time = '12:34:56'
set #date = #date + #time
select #date, DATEADD(DAY,-1,#date)
See SQL Fiddle
If the problem is that #date contains also time part, you can use:
set #date = convert(datetime, convert(date, #date)) + #time

Answer 6

Your code is correct.
DECLARE #date DATETIME = '1/1/2020'
DECLARE #time TIME = '1:00 pm'
DECLARE #Startdate DATETIME
SET #date = #date + #time
SELECT #Startdate = DATEADD(DAY, -1, #date)
#date = 2020-01-01 13:00:00.000
#Startdate = 2019-12-31 13:00:00.000
It isn't concatenating, it is adding them together. The time on #date is 0:00:00.000, so it might appear to be concatenating them. But change #date to '1/1/2020 1:00 am' and then:
#date = 2020-01-01 14:00:00.000
#Startdate = 2019-12-31 14:00:00.000