Calculate Daylight Savings time

Secondsunday - 2
secondsunday = LastDayOfWeek($firstdayofmarch);

Secondsunday - 3
secondsunday = LastDayOfWeek($firstdayofmarch)+Days(1);

Secondsunday - 4
secondsunday = LastDayOfWeek((LastDayOfWeek($firstdayofmarch)+Days(1)));

Then of course Secondsunday - final is your original full formula for secondsunday.

To me it looks like it is calculating correctly at the Secondsunday - 3 point, so basically changing the secondsunday part to be "LastDayOfWeek($firstdayofmarch)+Days(1);" seems to have fixed it for me in 2020, however your 2019 records will not calculate properly with this formula(it shows secondsunday as being 03/03/2019, which is wrong, should be 03/10/2019 ). Possibly other years as well. There must be a good solution here but I'm not sure what to tell you other than this will let you calculate correctly this year.

Also just so you are aware your current formula is also calculating the first Sunday of November incorrectly for 2020 as well but I imagine it is correct for 2019.

I do not know what you use this for but from my research QuickBase handles daylight savings time pretty well. It will be able to handle showing the correct times when a change has occurred even if the transition to daylight savings time happened during the event it is tracking time for(like start date and end date.) Just make sure your app is set up in the proper time zone already.