Convert from unix timestamp to datetime in sql ssms v15

If all your data within the range of int calculation (difference from two bigint numbers that falls in the range of int size), you can find a starting date and use the difference of this datetime and your data to calculate your target values.
Here is an unreliable solution but just an idea:

create table #temp (eventtimegmt bigint null )
 --CST -18000 seconds

 --

declare @startValue bigint =631173600000000 --get this manuallyfor --1990-01-01 
--https://www.epochconverter.com/
--This timestamp is in nanoseconds.covert it to second to remove 9 zeros

Insert into #temp values 
(636912338040000000),
(636912350040000000),
(636958101600000000),
(636912357240000000),
(636912359640000000 ) 

  Select eventtimegmt as  epochdtinNanoseconds,  
  DATEADD(second,(eventtimegmt-@startValue)/1000000000, CAST('1970-01-01 00:00:00' AS datetime)) as gmtdt 


  from #temp

 drop table #temp

Hi，@Edu_Mohd

Welcome to Microsoft T-SQL Q&A Forum!
The prerequisite for this is that the unix format must be correct.Try this:

SELECT columnA,Convert(varchar,DATEADD(MILLISECOND, CAST(RIGHT(dateTime, 3) AS INT)   
      - DATEDIFF(MILLISECOND,GETDATE(),GETUTCDATE()), DATEADD(SECOND, CAST(LEFT(dateTime, 10) AS INT), '1970-01-01')),111)as FormatedDate   
From mainTable where ...  

Test code：

create table #test01  
(  
 dateTime BIGINT  
)  
insert into #test01 values(1633418069),  
(1633504469),  
( 1633590869),  
(1665126869)  
  
SELECT Convert(varchar,DATEADD(MILLISECOND, CAST(RIGHT(dateTime, 3) AS INT)   
- DATEDIFF(MILLISECOND,GETDATE(),GETUTCDATE()), DATEADD(SECOND, CAST(LEFT(dateTime, 10) AS INT), '1970-01-01')),111)as FormatedDate from #test01  

Best regards,
Bert Zhou

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Let's say for example this unix timestamp: 636332338040000000 is recorded on 2021/10/5,

No, as others have pointed out you don't have a Unix timestamp if you get that value on that date. The value you posted can only be a Unix timestamp if we assume that it is nanoseconds, and then we end up in 1990. Well, we can make guesses like it's something like 0.01µs, but when I divide by 100 million, and get an int overflow which tells me that we are beyond 2037 which is when the Unix timestamps no longer will be 32-bit.

So you need to investigate whatever it is you are using to get these time stamps. But they are not Unix timestamps.