Translator-san has said there was a mistranslation, so it was Kouichi's birthday.
You can see the note posted at MRM.
The date went from 28 to 29, they were looking how the [birth]day was over.
Why did Mituru has to look at his phone when my Shougo baby is sleeping looking super sexy at their bed? (=・ω・=)?
Oh, I didn't know. Thanks!
It was like letting go. When mitsuru and kouchi were together probably mitsuru never missed greeting him at midnight. But then misturu did not message him on the current 29th and kouchi was waiting for mitsuru to greet him. i guess it's kind of a move on thing and a transition to a new story.
Oh, so that’s what it was... didn’t understand that page.
If you are wondering why Mitsuru and Koichi are looking at their phones and what this date mean i have an answer.
In first chapter Koichi tells Mitsuru that they can't meet. You can see a calendar with circled date ( http://iweb6.mangapicgallery.com/r/newpiclink/doronuma/1/f60fb25833d2c4232e5ecb53ff44f827.jpeg).
Some pages after we can see Mitsuru thougts: "We couldn't meet on my birthday because..."
So 28th November is a Mitsuru's birthday. They were looking at their phones because lots of people send their wishes at midnight to ones they like.