Adding to toinettec's comments, I sort of did this for the current Firecracker round. I was away with work and didn't connect my phone to hotel wifi. So although I started straight away, I didn't connect to internet till about 24 hours in. I'm currently in 6th place (although I was much lower when I woke up this morning) having collected every 2 hours all day today, and it's very competitive, so I think I'm on a leaderboard with people who started around the same time as me.
Thanks for the info. I have separate games going on two devices, so perhaps I will try to confirm for the sake of my own curiosity during the next mini-event.
For this one, I confirmed that starting at the same time on both devices put me on the same leaderboard.
(Though the odd thing is that even though it is clearly the same leaderboard on both devices based on the score counts, different usernames are listed. On my iPad, not connected to Facebook or GameCenter, rankings are currently:
1. Disney Villain Fan 41074 - 120
2. Glass Slippers 68054 - 104
3. Disney Villain Fan 49255 - 91
while on my iPhone, connected to my Facebook account, it's
1. schmoopsiekins - 120
2. Chonkinator - 104
3. NepetaStrider - 91
I assume this has to do with whether or not you are connected to Facebook, GameCenter, or neither, but I still found it peculiar!)