30 second offset with smalldatetime comparison on SQL

Question

I've came across an odd problem while comparing dates (actually smalldatetime) in SQL Server. It seems like there is a 30 seconds offset/bias while comparing dates close to the end of the day:

This is correct:

But this is definitely not correct:

What is happening here? Is math broken?

Answer 1

The smalldatetime type has a accuracy of one minute so the literal '2019-12-31 23:59:30' is rounded to '2020-01-01 00:00:00'.

Answer 2

If you read the [documentation][1] for smalldatetime, you will see this note:

https://learn.microsoft.com/en-us/sql/t-sql/data-types/smalldatetime-transact-sql?view=sql-server-2017

Accuracy One minute

In other words, the literal value is being rounded. As a result, what-you-see-is-not-what-you-get.