WTF?
So just how long have I been talking grollocks about not being able to run a Routine at the start of a time period?
It basically does the same as adding a separate Routine to run at the start time using the same conditions.
That’s a game changer isn’t it? The sort of thing to shout from the roof tops?
That’s quite useful!
At first I didn’t find the option, it’s inside the Time period trigger, right before the block with days of the week.
Absolutely. I had discovered this a few weeks ago, it actually works, and I was able to delete those stupid duplicates - I had routines like “between sunset and sunrise, if outside temperature drops below 14C, close all blinds” and then, because that wouldn’t work on a cold day when temperature was already below 14C at sunset, “at sunset, if outside temperature is below 14C, close all blinds”.
With that new switch, I kept the former, enabled “execute at start time”, and deleted the latter.
One of the things that has always made me crazy about SmartThings is that they don’t publish a changelog even to promote new features. 
Changelogs have generally gone out of style, at least with smartphone apps. It’s always “fixed bugs, added features”. Well duh, thanks Cpt. Obvious.
Philips Hue still has excellent changelogs, as does Apple for most of their official apps. Although come to think of it, they are usually called “release notes” now, I guess.
Yes, I created a Rule and read the JSON straight away to see what approach they used. So when I suggested it was basically the same as adding a second routine I did mean it literally.
