[Feature Request] A trimester class similar to `yearquarter` for academic terms.

[Steven314]

When dealing with data from academic institutions, I often find myself dealing with the three academic terms (or trimesters): fall, spring, and summer. I would find it useful if there was an option similar to yearquarter that allowed a cycle of 3 terms annually, rather than 4 quarters.

Something similar was brought up in {lubridate} in 2023: tidyverse/lubridate#1134, but never went anywhere. My reasoning for bringing it up here is to hopefully parallel the scale_*_yearquarter() functionality {tsibble} provides as well as the time series modeling in the rest of the tidyverts ecosystem.

I think the main advantage to having a new class over defining a four month interval would be in having a fiscal_start (or perhaps named academic_start) argument. That would allow the terms to be labelled according to their academic significance rather than their calendar names.
For instance, the institution I work for often defines the academic year beginning in the summer term (in May), so academic_start = 5 would be used.

Something like this:

tibble::tribble(
    ~calendar_year, ~month, ~academic_trimester,
    2025,           1,      '2025 T3',
    2025,           5,      '2026 T1',
    2025,           9,      '2026 T2'
) |> 
    dplyr::mutate(calendar_trimester = tsibble::make_yearmonth(calendar_year, month)) |> 
    tsibble::tsibble(index = calendar_trimester)
# # A tsibble: 3 x 4 [4M]
#   calendar_year month academic_trimester calendar_trimester
#           <dbl> <dbl> <chr>                           <mth>
# 1          2025     1 2025 T3                      2025 Jan
# 2          2025     5 2026 T1                      2025 May
# 3          2025     9 2026 T2                      2025 Sep

where the academic_trimester column would be of the class I'm proposing. In which case then, the index could be the academic trimester and the interval would be [1T].

I put together a start of what I'm looking for but I don't have it working yet: Steven314/tsibble ... yeartrimester.R, and a new trimester accessor: Steven314/tsibble ... trimester.R . I essentially copied the source for yearquarter and lubridate::quarter and adapted it.

[mitchelloharawild]

Thanks for the suggestion, I've written some thoughts in an issue of the {mixtime} repo for how this could be implemented more generally (mitchelloharawild/mixtime#16).

In short, I think that trimesters vary too much between usage to be added directly to tsibble, and instead it is better to make it easier to create custom time classes. So for now I suggest making a custom time class using tsibble with the code you've shared, and later I hope to make this process much easier.

The {mixtime} package is the planned replacement for yearweek(), yearmonth() and yearquarter() functions from tsibble. It is currently in the design stage, so there isn't much usable code yet -- however it is the perfect time to make requests and suggestions. The package is built around calendar structures and has ambition to create very flexible time classes (e.g. trimesters, working hours, trading days, ...).

[Steven314]

Those are good thoughts. I hadn't considered relating a day to a trimester since my use case only necessitates translating a coded string to its trimester, i.e. the spring 2025 term is depicted as "202510" in my data and relates to 2025 T3. I can see how having the ability to relate finer granularities would be useful for other applications.

I ended up starting work on a small separate package to suit my needs with trimesters.

[mitchelloharawild]

Looks great, feel free to ping me on GitHub if you have trouble with the tsibble code.