It's June 23, 2021 (Theory)
We all know that Dandadan began serialization in April 2021. Given this, it would be logical to assume that Momo and Okarun's adventure also started in that same year. There are additional details within the story that further support this idea. However, before delving into the specific evidence, we must first determine how many days have passed in Dandadan since the story began. To do this, I will use Ham_PHD’s timeline as a reference:
In chapter 25 of Dandadan you can see a partial view of a calendar in the school infirmary. Although you can't see the full calendar, you can note a few key details, particularly the fact that the month begins on a Saturday.
The month of this calendar began on Saturday.
And that coincides with May 2021.
We know that tomorrow there will be a supermoon. In 2021 only 4 supermoons occurred and the last one was on June 24, 2021, so Dandadan should be located on June 23, 2021.
Conclusions
The Dandadan manga was born in the year 2021 and it is reasonable to think that the story will take place in that year. It's been about a month since the events of the manga occurred. Taking into account the nursing calendar that begins on a Saturday (coinciding with May 2021). And that the last supermoon of 2021 was on June 24, 2021, it can be assumed that it is currently June 23, 2021. Although this is just a theory...