Get the max possible time of a date SQL Server

Question

What I am trying to do is take a date in SQL Server and find the last possible hour, minute, second, and millisecond of that date.

So if the date is this: 2021-02-16 13:08:58.620

I would like to return: 2021-02-16 23:59:59.999

I have tried something like this but it is not working and I guess that I am missing something where it is converting the time and keeping the hour, minute, second and millisecond of that date

Select 
    DateAdd(MILLISECOND, -1, (Convert(datetime, DATEADD(day, 1, DateValue), 101))) as lastPossibleDate
From 
    Table1

Answer 1

Turn it into a date to truncate the time, add a day, turn it back to a datetime, subtract a millisecond...

declare @Test datetime2(3) = '2021-02-16 13:08:58.620';

select dateadd(millisecond, -1, convert(datetime2(3),dateadd(day, 1, convert(date, @Test))));

Result
2021-02-16 23:59:59.999

Note if you use straight datetime rather than datetime(2) your accuracy is only down to 3ms. datetime(2) is the recommended datetime datatype to use.