Apply the Child Tax Credit family-element birth-date condition

changelog.d/2050.fixed.md

@@ -0,0 +1 @@
+- Restricted the Child Tax Credit family element to families responsible for a qualifying child or young person born before 6 April 2017, while preserving earlier supported years.

docs/book/programs/gov/dwp/tax-credits.md

@@ -59,11 +59,44 @@ benunit worked. PolicyEngine computes it via `ctc_entitlement`, gated by
 
 | Element | Parameter |
 |---------|-----------|
-| Family | `family_element.yaml` (extinct from 2017 for new claims) |
+| Family | `family_element.yaml` (one amount, subject to the birth-date condition below) |
 | Per qualifying child | `child_element.yaml` |
 | Disabled child addition | `dis_child_element.yaml` |
 | Severely disabled child addition | `severe_dis_child_element.yaml` |
 
+### Family element birth-date condition
+
+From 6 April 2017, the family element is included only if a claimant or
+joint claimant is responsible for a qualifying child or qualifying young
+person born **strictly before 6 April 2017**. A child born on that date
+does not qualify. One qualifying child is enough for the single family
+amount; an older adult or an excluded child cannot unlock it. The model
+uses its existing CTC child/QYP responsibility mask, including its existing
+looked-after-child and own-right-benefit approximations.
+
+`CTC_family_element` applies this condition separately from the individual
+child element's two-child limit. Its activation and cutoff parameters live
+under `child_tax_credit/eligibility/`; reforming them does not change the
+individual-element cutoff. The existing flat £545 amount is unchanged.
+Before the condition applies, supported model years retain the prior flat
+family amount. The false activation value at 2015-01-01 anchors model
+coverage, rather than statutory commencement. Annual parameters use the
+existing April sampling convention; this is not a part-year award model.
+
+The rule reuses `birth_day`: a supplied valid `date_of_birth` must agree
+with legal age on 6 October. A missing or zero date uses the existing
+annual age and months-since-birthday fallback; `birth_year` is not a
+replacement. Those conventions are not exact birth-date observations.
+Explicit family-element and final-amount inputs retain the ordinary model
+override contract and are not evidence of legal entitlement. Continuing
+claims, income tests, minimum awards, other elements and 2025 closure are
+unchanged; this update does not audit daily statutory award rounding.
+
+Historical coverage remains incomplete: this change addresses the
+supported 2015+ birth-date rule only. The pre-2011 baby addition and
+2003–2014 coverage remain follow-up work under issue #2050; no raw
+historical values or annual-conversion rules were extended.
+
 ### Two-child limit
 
 Since April 2017 the child element has been restricted to the first two

policyengine_uk/parameters/gov/dwp/tax_credits/child_tax_credit/eligibility/family_element_birth_date_condition_applies.yaml

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+description: Whether the Child Tax Credit family element requires a responsible child or qualifying young person born before the family-element cutoff. The false 2015 value anchors existing model coverage, not legal commencement.
+values:
+  2015-01-01: false
+  2017-04-06: true
+metadata:
+  unit: bool
+  period: year
+  label: Child Tax Credit family element birth date condition applies
+  reference:
+    - title: The Child Tax Credit (Amendment) Regulations 2017, regulations 1 and 4(b)
+      href: https://www.legislation.gov.uk/uksi/2017/387/regulation/4/made
+    - title: The Child Tax Credit (Amendment) Regulations 2017, regulation 1
+      href: https://www.legislation.gov.uk/uksi/2017/387/regulation/1/made

policyengine_uk/parameters/gov/dwp/tax_credits/child_tax_credit/eligibility/family_element_born_before.yaml

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+description: A responsible child or qualifying young person must be born strictly before this YYYYMMDD date for the Child Tax Credit family element, when its birth date condition applies.
+values:
+  2017-04-06: 20170406
+metadata:
+  unit: date
+  period: year
+  label: Child Tax Credit family element cutoff date of birth
+  reference:
+    - title: The Child Tax Credit Regulations 2002, regulation 7(2)(a)
+      href: https://www.legislation.gov.uk/uksi/2002/2007/regulation/7
+    - title: Tax Credits Act 2002, section 9(2)(a)
+      href: https://www.legislation.gov.uk/ukpga/2002/21/section/9

policyengine_uk/tests/policy/baseline/gov/dwp/tax_credits/ctc_family_element.yaml

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+# SI 2017/387 regs 1 and 4(b): strict responsible-child DOB condition.
+# Existing 2024 child element 3455; family element 545; zero-income continuing claims.
+- name: A child born on the family cutoff receives no family element
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 35
+        child_tax_credit_reported: 1
+      child:
+        age: 7
+        date_of_birth: 20170406
+    benunits:
+      family:
+        members:
+        - parent
+        - child
+        would_claim_uc: false
+    households:
+      household:
+        members:
+        - parent
+        - child
+  output:
+    child_tax_credit: 3455
+    tax_credits: 3455
+    CTC_family_element: 0
+    CTC_child_element: 3455
+- name: A child born the day before the family cutoff retains one family element
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 35
+        child_tax_credit_reported: 1
+      child:
+        age: 7
+        date_of_birth: 20170405
+    benunits:
+      family:
+        members:
+        - parent
+        - child
+        would_claim_uc: false
+    households:
+      household:
+        members:
+        - parent
+        - child
+  output:
+    child_tax_credit: 4000
+    tax_credits: 4000
+    CTC_family_element: 545
+    CTC_child_element: 3455
+- name: A child born after the family cutoff receives no family element
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 35
+        child_tax_credit_reported: 1
+      child:
+        age: 7
+        date_of_birth: 20170407
+    benunits:
+      family:
+        members:
+        - parent
+        - child
+        would_claim_uc: false
+    households:
+      household:
+        members:
+        - parent
+        - child
+  output:
+    child_tax_credit: 3455
+    tax_credits: 3455
+    CTC_family_element: 0
+    CTC_child_element: 3455