group-by a date column results in numerical group names

[daslu]

Zulip discussion:
https://clojurians.zulipchat.com/#narrow/stream/151763-beginners/topic/datetime.20problem.20in.20tablecloth.20group-by

Here is a self-contained example.
Create the file d.csv as follows:

date,x
2021-01-01,1
2021-01-02,2

Then:

(require '[tablecloth.api :as tc])

(-> "d.csv"
    (tc/dataset {:key-fn keyword})
    (tc/group-by :date))

_unnamed [2 3]:

| :name | :group-id |               :data |
|------:|----------:|---------------------|
| 18628 |         0 | Group: 18628 [1 2]: |
| 18629 |         1 | Group: 18629 [1 2]: |

The group names are turned into numbers.