Maximum difference between datetime and smalldatetime

Well! I learned how to solve this problem. I always thought that the difference between smalldatetime and datetime was not only due to the time when the valid time of smalldatetime was ~ The valid time of 2079/6/6 datetime ranges from 1753/1 to 1 ~ 9999/12/31, so I decided that smalldatetime would be used if the value does not need to be too long. But I ignored the difference that is more important, that is Smalldatetime can only be refined to minutes, while datetime can be refined to 3.33 milliseconds.Therefore, how can I save it? After the second is 00, it turns out that the original memory is smalldatetime. After I change the data type to datetime, the second part can be properly retained.