Question

In case of the SQL Server. Say, if I have a table with [myDate] column that is of the type datetime, what would adding 1 to it mean in the following context?

SELECT * 
  FROM [myTable] 
 WHERE [myDate] + 1 > @someDate

Answer 1

It adds one day (exactly 24 hours)

However, your query is more correct this way. The function or processing on the column usually invalidates index usage if there is one of that column

WHERE [myDate] > @someDate - 1

OR

WHERE [myDate] > DATEADD(day, -1, @someDate)