multi-calendar-engine

This library is used to work with dates in multiple calendrical system (i.e. Ethiopic, Nepali etc..) across DHIS-2 applications. It mainly exposes two components:

1. Hooks like useDatePicker used to build UI components (part of @dhis/ui) such as the Calendar and CalendarInput components.
2. Helper methods, like generateFixedPeriods and getNowInCalendar to deal with period generations, date manipulation and arithmetic across multiple calendrical systems.

The idea behind this library is to abstract the complicated details of dealing with dates in DHIS2, and to centralise them in one place for consumption in different apps and libraries.

Internally, the library relies on the Temporal API which aims to achieve, among other goals, full-support for non-Gregorian calendars.

This document has some of the requirements and design decisions for this project.

This Jira epic lists the app that moved to using the library, and what comes next.

Periods and Dates helpers

The library also provides helper methods to work with periods and dates in multiple calendars. This allows doing date arithmetic, comparisons in a standard timezone-safe way. It currently relies on the Temporal API.

The library is built on top of Temporal API, but this is an implementation detail, and one of the goals of the library is to hide that detail to allow replacing Temporal with a different implementation in the future, i.e. a Kotlin multi-platform implementation to achieve consistency between web, android and backend.

generateFixedPeriods

generateFixedPeriods returns the periods of a specific type in a specific year. This is used, for example in Data Visualizer, to display the list of periods of different types (i.e. monthly, weekly, yearly) in a specific year, according to the user's calendar and locale.

Examples

Calling generateFixedPeriods({year: 2015, periodType: "FYNOV", calendar: "gregory"}) will return all the November financial year periods (FYNOV) from the year 2015 (defaults to 10 years). Here are some more examples:

// some examples
import { generateFixedPeriods } from '@dhis2/multi-calendar-dates'

describe('generateFixedPeriods', () => {
    it('should generate financial years periods (starting November) for the specified', () => {
        const result = generateFixedPeriods({
            year: 2015,
            calendar: 'gregory',
            locale: 'en',
            periodType: 'FYNOV',
        })

        expect(result).toEqual(
            expect.arrayContaining([
                {
                    id: '2015Nov',
                    name: 'November 2015 - October 2016',
                    startDate: '2015-11-01',
                    endDate: '2016-10-31',
                },
                {
                    id: '2014Nov',
                    name: 'November 2014 - October 2015',
                    startDate: '2014-11-01',
                    endDate: '2015-10-31',
                },
                // .... up to
                {
                    id: '2006Nov',
                    name: 'November 2006 - October 2007',
                    startDate: '2006-11-01',
                    endDate: '2007-10-31',
                },
            ])
        )
    })
    it('should generate weekly periods (weeks starting on Sunday) for the specified year', () => {
        const result = generateFixedPeriods({
            year: 2015,
            calendar: 'gregory',
            locale: 'en',
            periodType: 'WEEKLYSUN',
        })

        expect(result).toEqual(
            expect.arrayContaining([
                {
                    id: '2015SunW1',
                    iso: '2015SunW1',
                    name: 'Week 1 - 2015-01-04 - 2015-01-10',
                    startDate: '2015-01-04',
                    endDate: '2015-01-10',
                },
                // .... up to
                {
                    id: '2015SunW52',
                    iso: '2015SunW52',
                    name: 'Week 52 - 2015-12-27 - 2016-01-02',
                    startDate: '2015-12-27',
                    endDate: '2016-01-02',
                },
            ])
        )
    })

    it('should generate monthly periods (localised in Spanish) for the specified year', () => {
        const result = generateFixedPeriods({
            year: 2015,
            calendar: 'gregory',
            locale: 'es',
            periodType: 'MONTHLY',
        })

        expect(result).toEqual(
            expect.arrayContaining([
                {
                    id: '201501',
                    iso: '201501',
                    name: 'Enero de 2015',
                    startDate: '2015-01-01',
                    endDate: '2015-01-31',
                },
                // .... up to
                {
                    id: '201512',
                    iso: '201512',
                    name: 'Diciembre de 2015',
                    startDate: '2015-12-01',
                    endDate: '2015-12-31',
                },
            ])
        )
    })

    it('should generate bi-monthly periods for the Ethiopic calendar for the specified year', () => {
        const result = generateFixedPeriods({
            year: 2015,
            calendar: 'ethiopic',
            locale: 'en',
            periodType: 'BIMONTHLY',
        })

        expect(result).toEqual(
            expect.arrayContaining([
                {
                    id: '201501B',
                    iso: '201501B',
                    name: 'Meskerem - Tekemt 2015',
                    startDate: '2015-01-01',
                    endDate: '2015-02-30',
                },
                // .... up to
                {
                    endDate: '2015-12-30',
                    id: '201506B',
                    iso: '201506B',
                    name: 'Hamle - Nehasse 2015',
                    startDate: '2015-11-01',
                },
            ])
        )
    })
    it('should generate six-monthly periods for the Nepali calendar for the specified year', () => {
        const result = generateFixedPeriods({
            year: 2078,
            calendar: 'nepali',
            locale: 'en',
            periodType: 'SIXMONTHLY',
        })

        expect(result).toEqual([
            {
                id: '2078S1',
                iso: '2078S1',
                name: 'Baisakh - Ashwin 2078',
                startDate: '2078-01-01',
                endDate: '2078-06-31',
            },
            {
                id: '2078S2',
                iso: '2078S2',
                name: 'Kartik - Chaitra 2078',
                startDate: '2078-07-01',
                endDate: '2078-12-30',
            },
        ])
    })
})

Types

The method takes a single options parameter and returns an Array<FixedPeriod>.

The options param

The options param is an object of type GeneratedPeriodParams:

type GeneratedPeriodParams = {
    year: number
    periodType: PeriodIdentifier
    calendar: SupportedCalendar
    locale?: string
    startingDay?: number /** 1 is Monday */
    yearsCount?: number
}

For convenience, locale can be passed in the Java-like style (i.e. ar_SD rather than ar-SD) and it will be converted to the JS-style. yearsCount is used when generating yearly periods to know how many years to generate (defaults to 10). `

periodType can be one of the period identifiers defined here. Although the library internally is flexible and can accept, for example, quarterly periods starting any month (i.e. QUARTERLYMAY), support for such periods might not be available in the backend, so they should be used carefully. The flexibility, though, means that a new period could be added to the backend and used without changes in the frontend.

The return value

The returned value is an array of FixedPeriods:

type FixedPeriod = {
    id: string
    iso?: string
    name: string
    startDate: string
    endDate: string
}

• id (and iso) are the same right now, and return a period identifier, i.e. 2015Q1 (quarter one of the year 2015) or 2015SunW1 (the first week - starting Sunday - of the year 2015). The full list of values are documented in periodValues in the feature files.

please use id to identify the period. iso was added for backwards compatibility with existing implementations

• startDate and endDate are the start and end dates of the specific period. This is provided for convenience, but the backend currently only makes use of the ID to work out what each period means.

• name is the human readable name of the period, according to what we dispaly in DHIS2 Data Visualizer, i.e. Week 1 - 2014-13-03 - 2015-01-04 for a weekly period or 2015 for a yearly period.

getNowInCalendar

getNowInCalendar returns today's date in the specified calendrical system.

Examples

// Assuming today's date is 13th October 2021
beforeEach(() => {
    // 13 October 2021 UTC
    jest.spyOn(Date, 'now').mockReturnValue(1634089600000)
})
it('should get today date in Gregorian', () => {
    const { day, month, year } = getNowInCalendar('gregory', 'UTC')
    expect({ day, month, year }).toEqual({ day: 13, month: 10, year: 2021 })
})
it('should get today date in Ethiopic', () => {
    const { day, month, eraYear: year } = getNowInCalendar('ethiopic', 'UTC')
    expect({ day, month, year }).toEqual({
        day: 3,
        month: 2,
        year: 2014,
    })
})

Types

The method takes two positional arguments:

• calendarToUse which can be one of the calendars specified here (defaults to gregory calendar if not specied.
• timeZone: a string representing the time zone (defaults to UTC) of the user.

and returns a ZonedDateTime object, which can be destructured to .year or .eraYear (eraYear preferred to avoid an issue with Ethiopic calendar), .month, .day which returns the values in the specified calendar, or you can .getISOFields() to return the underlying iso8601 (gregory) date.

ToDo: we should also return the date stringified into yyyy-MM-dd since this is the most common usecase and it would help clients not to have to do the conversion manually.

Upcoming methods

There are new methods that will be added soon (in the alpha release right now), such as getFixedPeriodByDate (which, given a date in a calendar, it returns what fixed period the date falls in) and getAdjacentFixedPeriods (that facilitates being able to navigate to next and previous periods).

These new methods will be documented when merged into main branch, but this gives an idea about the general approach we want to follow with the library; if you encounter a use case that you think it should be included in the library then please get in touch to discuss whether it should live there, or at the consumer level.

Special cases and considerations with periods logic

There are some special DHIS2-specific cases when doing period calculations that we handle. The specs for these are documented in the tests and cucumber feature files. Some of these are:

• Dealing with the 13th month in the Ethiopian calendar: the Ethiopic calendar has 13 months: 12 months of 30 days, then the 13th month has 5 or 6 days depending on whether it's a leap year or not. When generating periods for bi-weekly or less (bi-weekly, weekly, daily) then the 13th month is displayed. But when we are generating periods for monthly or larger, we do not display the 13th month. The backend then makes the decision about where to include any data for the 13th month, so when doing monthly analytics, the 13th month will be lumped with the following month's data.

• The period IDs are dictated by the backend, and they use the Gregorian month names, for example, QuarterlyNov or QuarterlyApr. In the frontend, in a non-Gregorian calendar, Nov will be "translated" into the 11th month of the current calendar system (Hamle in Ethiopic). Ideally, we will have period IDs that are calendar-agnostic in the future, but this works for now as a solution.

• From the frontend perspective, we always send a date in the calendar system. It is formatted in an ISO-like format (yyyy-MM-dd). It is "ISO-like", because the date is actually in the calendar system rather than the Gregorian ISO8601 calendar. So when a system set to Ethiopic, sends 2015-01-01 to the backend, it means the year 2015 in Ethiopic (which is 2022 in Gregorian). This is what the backend expects, and if there is a need for a conversion, then it happens in the backend.

  • Given we have the Temporal object in the frontend, we have the ability to send the date converted into Gregorian, but that's what the backend expects right now.

• Nepali is implemented as a custom calendar: see nepaliCalendar.ts. Nepali is luni-solar calendar which means that the length of months can vary from year to year. The way Temporal does date arithmetic, is that it converts the day to gregorian, performs the arithmetic, then converts it back to the calendar. This leads to weird situations where you do nepaliDate.add({month: 1}) for example, but the result is still in the current month (since the month has 30 days in Gregorian, but 32 in Nepali for example). To get around that, we set the day to the 14th before doing arithmetic (in the context of period generation) and this leads to results that make sense in the context of DHIS2.

• Nepali is also not supported as a locale in major browsers. So we rely on localising it with a hardcoded map, meaning we don't have the same flexibility showing the day or month names as short or long for example with .toLocaleString. That means that the consumer can only use one of ne-NP or en-NP (nepali months and days transliterated into English) as a locale with the Nepali calendar, and they can't use browser localisations (so they can't display Nepali month names in French for example, but they would be able to do so with Ehtiopic or Islamic calendars).

• In Ethiopic calendar, when consuming a date, use .eraYear rather than .year to get the year part of a date. There is an ongoing discussion on Temporal to decide which era of the Ethiopic calendar should be the default, but it's likely to be browser-dependent at the end, so it's safer to use eraYear as this is what users in Ethiopia would expect to see. We looked at abstracting this difference away in the library, but there was no easy solution for it.

• The library also abstracts some DHIS2-specific inconsistencies with the backend to make it easier for consumers of the library. For example, it has a map to convert from the calendar IDs used in DHIS2 to the ones expected by the library: dhis2CalendarsMap.ts. This maps "ethiopian" (used in DHIS2) to "ethiopic" which is the standard ID used in JavaScript (in Unicode CLDR). It also maps the Java locale names with an underscore to ones with a dash, to make them easier to use in a JavaScript context (i.e from ar_SD to ar-SD).

Hooks for Calendar UI

The library provides a hook useDatePicker that's consumed by the dhis/ui calendar components. There are two components that currently use it:

• Calendar: a calendar component supporting non-Greogorian calendars
• CalendarInput: a wrapper around the Calendar component to support the most common use case for a calendar when we want to display it next to an input.

useDatePicker takes a DatePickerOptions object, and returns UseDatePickerReturn that contains information needed to render a UI component, i.e. the localised day names, and the weeks in the current view (month).

// the options passed to useDatePicker
type DatePickerOptions = {
    date: string
    options: PickerOptions
    onDateSelect: ({
        calendarDate,
        calendarDateString,
    }: {
        calendarDate: Temporal.ZonedDateTime
        calendarDateString: string
    }) => void
}

// the return type of the hook
type UseDatePickerReturn = UseNavigationReturnType & {
    weekDayLabels: string[]
    calendarWeekDays: {
        zdt: Temporal.ZonedDateTime
        label: string | number
        calendarDate: string
        onClick: () => void
        isSelected: boolean | undefined
        isToday: boolean
        isInCurrentMonth: boolean
    }[][]
}

Architecture Design Records

• Use Temporal API as the backbone of the engine