feat: add bieannial and triannial billing frequency

[HenningWendtland]

Addition of biennial and triennial billing (every 2 and 3 years).
This was easily possible for the period_type == "start date" but not for "calendar months" type, so just implemented for the first type.
To improve the UI the correct types are displayed dynamically

Reference: AIT-46

[greptile-apps]

Confidence Score: 5/5

The core period-computation logic is correct, well-tested, and backed by unit tests that cover both period types and edge cases around block boundaries.

The frequency arithmetic and billing-block alignment are mathematically sound, and the new test cases verify the boundary behaviour. The two noted concerns — a stale translation key and the first-block skip for mid-year CalendarMonths subscriptions — are non-blocking: one is a localisation quality issue and the other mirrors pre-existing behaviour for Yearly subscriptions.

Translation files (de.po / main.pot) should be regenerated after aligning the error message string in simple_subscription.py.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[get_from_and_to_date] --> B{period_type?}
    B -->|StartDate| C{billing_time?}
    B -->|CalendarMonths| D{billing_time?}
    C -->|AtBeginning| E[get_date_period\neval_date]
    C -->|AfterEnd| F[get_date_period\ncurrent_start - 1 day]
    D -->|AtBeginning| G[get_calendar_period\neval_date]
    D -->|AfterEnd| H[get_calendar_period\ncurrent_start - 1 day]
    G --> I{frequency in\nMULTI_YEAR?}
    H --> I
    I -->|No| J[INVOICE_MONTH_MAP\nlookup → from_date]
    I -->|Yes| K[block_start_year =\nstart_date.year +\nfloor div aligned span]
    K --> L[from_date = Jan 1\nof block_start_year]
    J --> M[to_date = from_date\n+ frequency months - 1 day]
    L --> M
    E --> N[months = frequency.value\nrelativedelta arithmetic]
    F --> N

Loading

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
flowchart TD
    A[get_from_and_to_date] --> B{period_type?}
    B -->|StartDate| C{billing_time?}
    B -->|CalendarMonths| D{billing_time?}
    C -->|AtBeginning| E[get_date_period\neval_date]
    C -->|AfterEnd| F[get_date_period\ncurrent_start - 1 day]
    D -->|AtBeginning| G[get_calendar_period\neval_date]
    D -->|AfterEnd| H[get_calendar_period\ncurrent_start - 1 day]
    G --> I{frequency in\nMULTI_YEAR?}
    H --> I
    I -->|No| J[INVOICE_MONTH_MAP\nlookup → from_date]
    I -->|Yes| K[block_start_year =\nstart_date.year +\nfloor div aligned span]
    K --> L[from_date = Jan 1\nof block_start_year]
    J --> M[to_date = from_date\n+ frequency months - 1 day]
    L --> M
    E --> N[months = frequency.value\nrelativedelta arithmetic]
    F --> N

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_{Reviews (3): Last reviewed commit: "feat: implement biennial and trieannial ..." | Re-trigger Greptile}

[HenningWendtland]

@greptileai

[HenningWendtland]

@greptileai

[barredterra]

One last nitpick

[barredterra]

Suggested change

	def get_calendar_period(
	eval_date: date, frequency: Frequency, start_date: date | None = None
	) -> tuple[date, date]:
	"""Return the first and last day of the calendar period containing `eval_date`."""
	if frequency in MULTI_YEAR_FREQUENCIES and not start_date:
	frappe.throw(
	_("Start Date is required for frequency {0}.").format(
	_(frequency.name, context="Frequency of Subscription")
	)
	)
	if frequency in MULTI_YEAR_FREQUENCIES:
	years_span = frequency.value // 12
	def get_calendar_period(
	eval_date: date, frequency: Frequency, start_year: int | None = None
	) -> tuple[date, date]:
	"""Return the first and last day of the calendar period containing `eval_date`."""
	if frequency in MULTI_YEAR_FREQUENCIES:
	if not start_year:
	raise ValueError("start_date is required for multi-year frequencies")
	years_span = frequency.value // 12

• This method only needs the start year, not the entire start date (less for the reader to think about)
• Calendar utils were meant to be independent of frappe, so they should raise regular python exceptions instead of frappe.throw(). A framework-caller can then catch these and frappe.throw(). This keeps the separation between pure date math and framework clean.