P3: depreciation straight-line by days through the final month, schedules by day (defects 22, 9, D-37)

The final month takes what the months before leave, spread over its days;
a disposed asset stops on its disposal day. A skipped month is caught up
in the month the run is made; a reversed run un-posts its month. New
schedules spread by day; equal-months schedules must start on the 1st.
Amounts round to the currency's own minor unit. Kani and Lean proofs; an
exhaustive search over runs, reversals and disposals.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

examples/depreciation-final-month-drift.toml

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+id = "depreciation-final-month-drift"
+title = "The final month of a mid-month life takes what the months before it leave, evenly over its days"
+rule = "When a life does not start on the 1st, its final month is charged the depreciable amount less what the months before it took, spread evenly over the final month's days."
+status = "CONVENTION"
+sources = ["ASC 360-10-35-4"]
+tags = ["depreciation", "straight-line", "final-month", "month-length-drift", "p3-defect-22"]
+
+[setup]
+currency = "USD"
+fiscal_year_start = "01-01"
+basis = "accrual"
+rounding = "half-away-from-zero"
+
+[[setup.accounts]]
+code = "1000"
+name = "Bank"
+type = "asset"
+
+[[setup.accounts]]
+code = "1500"
+name = "Equipment"
+type = "asset"
+
+[[setup.accounts]]
+code = "1510"
+name = "Accumulated Depreciation"
+type = "asset"
+
+[[setup.accounts]]
+code = "3000"
+name = "Capital"
+type = "equity"
+
+[[setup.accounts]]
+code = "6200"
+name = "Depreciation Expense"
+type = "expense"
+
+# Cost 1,030.00, salvage 100.00 (930.00 to depreciate), 3 months from
+# 2026-01-15: the life is 2026-01-15 through 2026-04-14.
+[[setup.assets]]
+id = "D"
+description = "Asset D"
+cost = "1030.00"
+salvage = "100.00"
+life_months = 3
+in_service = "2026-01-15"
+method = "straight_line"
+cost_account = "1500"
+accumulated_account = "1510"
+expense_account = "6200"
+
+[[events]]
+date = "2026-01-15"
+description = "Capital"
+lines = [
+  { account = "1000", side = "debit", amount = "1030.00" },
+  { account = "3000", side = "credit", amount = "1030.00" },
+]
+
+[[events]]
+date = "2026-01-15"
+description = "Bought asset D"
+lines = [
+  { account = "1500", side = "debit", amount = "1030.00" },
+  { account = "1000", side = "credit", amount = "1030.00" },
+]
+
+# Hand check (worked without the code): 930.00 / 3 = 310.00 a month.
+# January holds 17 of its 31 days: 310.00 x 17/31 = 170.00. February and
+# March are whole: 310.00 each. Before April: 790.00. April holds 14 days
+# (1-14) and is charged what is left, 930.00 - 790.00 = 140.00, evenly:
+# 10.00 a day (January's own daily rate, 310.00/31). Through Apr 13:
+# 130.00; accumulated 920.00; book value 1,030.00 - 920.00 = 110.00.
+[[expect]]
+query = "depreciation_schedule"
+asset = "D"
+as_of = "2026-04-13"
+
+[expect.values]
+"2026-01" = "170.00"
+"2026-02" = "310.00"
+"2026-03" = "310.00"
+"2026-04" = "130.00"
+accumulated = "920.00"
+book_value = "110.00"
+
+# Through Apr 14, the life's last day: April 140.00, accumulated 930.00,
+# book value 100.00 (the salvage, never depreciated).
+[[expect]]
+query = "depreciation_schedule"
+asset = "D"
+as_of = "2026-04-14"
+
+[expect.values]
+"2026-04" = "140.00"
+accumulated = "930.00"
+book_value = "100.00"