I did it online and it was easy (I did this from my iPhone), same as you, didn't want yet another app on my phone....

We also did online and not on the app - very easy. Although had to do it more than expected. Once for arrival from LHR as planned, then after going airside at SIN a couple of days later our flight to TPE was cancelled 1-2hrs post scheduled departure time and so had to hurriedly complete again to re-enter Singapore, before then leaving again 24hrs later.

We got taken aside by border control on the re-entry who unsurprisingly wanted to know why after spending 2 days in Singapore we then exited and tried to re-enter within the space of a few hours but all fine when explained, and no issue on the re-entry from TPE a week later (when of course we did the cards again).

The app is very easy to use. I wanted also to avoid a new app on my phone, but sincerely...

Helpful for frequent travellers, but if its a one off visit, can still do on-line in your phone. Mrs. M and I frequently forget and do it wile taxiing to the gate.

It is also an offence to go air side if not intending to travel. For example if you want to meet someone connecting through Singapore but they don't want to clear immigration, or couldn't, in the old days you could buy an economy ticket to KUL, go airside, meet them, and then return to immigration and say you had changed your mind. I believe its a fine of up to $20,000, imprisonment for up to two years, or both. My guess is that immigration only wanted to check there was a legitimate reason for not boarding the flight.

Thanks yep makes sense. They'd been told about the cancelled flight so on seeing our boarding passes and asking our new plans they were fine.

Has anyone arrived on the wrong arrival day and knows what happens?

My flight is scheduled to arrive at 00:10 into SIN. I'll put the correct arrival date in, but... the flight is regularly 30+ minutes early so typically arrives the previous day. Might I have to loiter around and wait for midnight because my arrival card will technically be for the next day?

Just edit or resubmit once you land.