I did a search on the topic "oneshot" and found two posts. These helped me square away potential issues I might have had on when to calculate DST (as well as other features in my code).
The first is: oneShotGuard
https://community.particle.io/t/doing-something-at-a-certain-time/40245/5
I took the time to implement what @ScruffR said about it. It worked great for me.
The second is: class OneShot
https://community.particle.io/t/water-tank-monitor-using-hall-effect-switches/24868/26
Then, I borrowed from @Ric and made my own small library. Once done, it was relatively easy to implement oneShotGuard's for many different tasks that I had going and be confident they would fire once and only once, at the appropriate time.
BTW, thanks to all of you for the contributions! 