What feature are you suggesting?

1- Need a GUI.
For 1.12, I used InGame Info XML
display the season, and sub-season. It was convenient. For 1.15, this mod has not been updated and is hardly going to be. Графический интерфейс всегда удобный и простой способ увидеть, что вам нужно.

2- Improvement of the calendar is possible.
The season sensor is not very easy to use, the calendar is more convenient, but it's just an animated piece of paper, I always forget the pictures of the seasons, and I was foolishly trying to figure out what time of year it was. This can be done, for example, when you hover over a framed calendar (by default season 1 is 24 days), we see summer - 20 Day.

3- New block with accurate indicators of the season and subseason. Mb mechanical calendar for a year or for a separate season with a sub-season. Thanks to the internal interface, a full-fledged calendar with display of the season and dates, as well as a highlighted today's date in the game. For example, something like a painting from a vanilla , in its usual position shows the season and the date when you open the full calendar with the dates of the season (3 sub-seasons). Or a calendar with sub-seasons in normal position and an open full calendar of 4 seasons and 12 sub-seasons.
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Perhaps for option 3, you can make a limitation in configs from 1 to 30 days per subseason and add the ability to customize each subseason from 1 to 30 days.

Ps/
The mod is excellent, I hope that in the future it will be updated and improved.

But at the moment I don't like the display of seasons. More precisely, it simply does not exist. You look at the sensor, well, it's a redstone lantern somewhere, yes, summer, and what date? how much until spring? Or the same calendar, showing a picture that you always forget. You need something complete to display.

We're not adding super in-depth season displays, especially through the GUI. If other people want to make add-ons that do that, they're more than welcome to, but that's just not the style of Minecraft, and we like to try sticking to vanilla designs as much as possible.

I think the calendar is more than sufficient enough to tell what season it is and how far along into the season it is (Which is what the dots are for). Obviously it doesn't show the exact day it is in the season, but personally I like the idea of keeping track of it yourself, which is easy enough since the default season cycle now takes 8 days per sub season, which matches the moon phases. The exact day it is in the season really isn't that important either. Crops aren't going to die as soon as the season changes or anything (And by default now, they just grow slowly out of season rather than not growing at all).

Also no offense, but I feel like the season icons are more than clear enough that anyone should be able to understand them. Maybe it's a cultural thing, but like, what else would you match a snowflake to other than winter, or a falling leaf to autumn, and so on?