Who Stuck These Letters In My DateTimes?

YYYY-MM-DD hh:mm:ss[.fractional seconds]

YYYY-MM-DDTHH:mm:ss.sssZ

DECLARE @jsonData nvarchar(max) = N'{ "createDate" : "2017-03-28T12:45:00.000Z" }'
-- SQL's JSON_VALUE() will read in the JSON date time as a string
SELECT JSON_VALUE(@jsonData, '$.createDate')
-- Output: 2017-03-28T12:45:00Z
-- If we want to read it in as a SQL datetime2, we need to use a CONVERT() (or a CAST())
SELECT CONVERT(datetime2, JSON_VALUE(@jsonData, '$.createDate'))
-- Output: 2017-03-28 12:45:00.0000000
-- 7 zeroes after the decimal? Our source only had 3 zeroes!
-- Since JSON/JavaScript times have decimal precision to only 3 places, we need to make
-- the precision of datetime2 match
SELECT CONVERT(datetime2(3), JSON_VALUE(@jsonData, '$.createDate'))
-- Output: 2017-03-28 12:45:00.000
-- So now we are returning our UTC date time from JSON, but what if we need to convert it to a different time zone?
-- Using SQL Server 2016's AT TIME ZONE with CONVERT() will allow us to do that easily.
-- To get a full list of time zone names, you can use SELECT * FROM sys.time_zone_info
SELECT CONVERT(datetime2(3), JSON_VALUE(@jsonData, '$.createDate')) AT TIME ZONE 'Eastern Standard Time'
-- Output: 2017-03-28 12:45:00.000 -04:00
-- What if we just need to grab the date?  Pretty easy, just CONVERT() to date
SELECT CONVERT(date, JSON_VALUE(@jsonData, '$.createDate'))
-- Output: 2017-03-28
--Same with just the time, just remember to use a precision value of 3
SELECT CONVERT(time(3), JSON_VALUE(@jsonData, '$.createDate'))
-- Output: 12:45:00.000

DECLARE @sqlData datetime2 = '2017-03-28 12:45:00.1234567'
-- Let's first try the simplest SQL to JSON conversion first using FOR JSON PATH
SELECT @sqlData as SQLDateTime2 FOR JSON PATH
-- Output: [{"SQLDateTime2":"2017-03-28T12:45:00"}]
-- Honestly that's not too bad!
-- The datetime gets created in the YYYY-MM-DDTHH:MM:SS.fffffff format
-- Although this is pretty much what we need, what if we want to be explicit and specify that we are in UTC?
-- Just add the AT TIME ZONE modifier and we will get our JSON "Z" indicating UTC
SELECT @sqlData AT TIME ZONE 'UTC' AS SQLDateTime2 FOR JSON PATH
-- Output: [{"SQLDateTime2":"2017-03-28T12:45:00.1234567Z"}]
-- And if we provide a different time zone offset, the JSON is formatted correctly with the +/-HH:MM suffix:
SELECT @sqlData AT TIME ZONE 'Eastern Standard Time' AS SQLDateTime2 FOR JSON PATH
-- Output: [{"SQLDateTime2":"2017-03-28T12:45:00.1234567-04:00"}]
-- You might notice that there are 7 fractional second decimal places in all of the above examples.
-- Although out of JSON spec, this is ok!
-- What if we just want to insert the date?  Just specify with a SQL CONVERT()
SELECT CONVERT(date, @sqlData) as SQLDateTime2 FOR JSON PATH
-- Output: [{"SQLDateTime2":"2017-03-28"}]
-- And the same goes with the time portion
SELECT CONVERT(time, @sqlData) as SQLDateTime2 FOR JSON PATH
-- Output: [{"SQLDateTime2":"12:45:00.1234567"}]

DECLARE @sqlDate datetime2 = '2017-03-28 12:45:00.1234567'
DECLARE @jsonData nvarchar(max) = N'{ "createDate" : "2017-03-28T12:45:00.000Z" }'
        ,@newDate datetime2(3) = '2017-03-28T12:48:00.123Z'
-- Let's start out modifying our data by replacing the value completely
SELECT JSON_VALUE(@jsonData, '$.createDate')
-- If we want to pass in a perfectly formatted JSON string, then it's pretty easy
SELECT JSON_MODIFY(@jsonData, '$.createDate', '2017-03-28T12:48:00.123Z')
-- Output: { "createDate" : "2017-03-28T12:48:00.123Z" }
-- If we want to pass in a SQL datetime2 value, say like what we have stored in @newDate, then things get a little messy.
-- The JSON_MODIFY function requires the third argument to be the nvarchar datatype.  This means
-- we need to get our SQL datetime2 into a valid JSON string first.  
-- If we use FOR JSON PATH to create the JSON date from the SQL datetime2, things get ugly because 
-- FOR JSON PATH always creates a property : value combination
SELECT JSON_MODIFY(@jsonData, '$.createDate', (SELECT @newDate as newDate FOR JSON PATH))
-- Output: { "createDate" : [{"newDate":"2017-03-28T12:48:00.123"}] }
-- In order to only pass the JSON datetime into the value for the "createDate" property, we need to 
-- use the CONVERT style number 127 to convert our dateTime to a JSON format
SELECT JSON_MODIFY(@jsonData, '$.createDate', (SELECT CONVERT(nvarchar, @newDate, 127)))
-- Output: { "createDate" : "2017-03-28T12:48:00.123" }
-- But what happened to our "Z" indicating UTC?  
-- We of course need to specify the AT TIME ZONE again:
SELECT JSON_MODIFY(@jsonData, '$.createDate', (SELECT CONVERT(nvarchar, @newDate AT TIME ZONE 'UTC', 127)))
--Output: { "createDate" : "2017-03-28T12:48:00.123Z" }