diff --git a/app/services/supplementary-billing/determine-charge-period.service.js b/app/services/supplementary-billing/determine-charge-period.service.js
new file mode 100644
index 0000000000..61b25a5a9f
--- /dev/null
+++ b/app/services/supplementary-billing/determine-charge-period.service.js
@@ -0,0 +1,64 @@
+'use strict'
+
+/**
+ * Determines what the charge period is for a charge version in a given billing period
+ * @module DetermineChargePeriodService
+ */
+
+/**
+ * Returns a charge period, which is an object comprising `startDate` and `endDate`
+ *
+ * The charge period is determined as the overlap between the charge version's start and end dates, and the financial
+ * year. So the charge period's start date is the later of the two's start dates, and the charge period end date is the
+ * earlier of the two's end dates.
+ *
+ * > Charge versions may not have an end date; in this case, we simply use the financial year end date.
+ *
+ * @param {module:ChargeVersionModel} chargeVersion the charge version being processed for billing
+ * @param {number} financialYearEnding the year the financial billing period ends
+ *
+ * @returns {Object} the start and end date of the calculated charge period
+ */
+function go (chargeVersion, financialYearEnding) {
+  const financialYearStartDate = new Date(financialYearEnding - 1, 3, 1)
+  const financialYearEndDate = new Date(financialYearEnding, 2, 31)
+
+  if (_periodIsInvalid(chargeVersion, financialYearStartDate, financialYearEndDate)) {
+    throw new Error(`Charge version is outside billing period ${financialYearEnding}`)
+  }
+
+  const chargeVersionStartDate = chargeVersion.startDate
+  const latestStartDateTimestamp = Math.max(financialYearStartDate, chargeVersionStartDate)
+
+  // If the charge version has no end date then use the financial year end date instead
+  const chargeVersionEndDate = chargeVersion.endDate || financialYearEndDate
+  const earliestEndDateTimestamp = Math.min(financialYearEndDate, chargeVersionEndDate)
+
+  return {
+    startDate: new Date(latestStartDateTimestamp),
+    endDate: new Date(earliestEndDateTimestamp)
+  }
+}
+
+/**
+ * Determine if the charge version starts after or ends before the billing period
+ *
+ * We never expect a charge version outside the financial period but we do this just to ensure we never return
+ * nonsense and all possible scenarios are covered in our tests 😁
+ *
+ * @param {Object} chargeVersion chargeVersion being billed
+ * @param {Date} financialYearStartDate billing period (financial year) start date
+ * @param {Date} financialYearEndDate billing period (financial year) end date
+ *
+ * @returns {boolean} true if invalid else false
+ */
+function _periodIsInvalid (chargeVersion, financialYearStartDate, financialYearEndDate) {
+  const chargeVersionStartsAfterFinancialYear = chargeVersion.startDate > financialYearEndDate
+  const chargeVersionEndsBeforeFinancialYear = chargeVersion.endDate && chargeVersion.endDate < financialYearStartDate
+
+  return chargeVersionStartsAfterFinancialYear || chargeVersionEndsBeforeFinancialYear
+}
+
+module.exports = {
+  go
+}
diff --git a/app/services/supplementary-billing/format-sroc-transaction-line.service.js b/app/services/supplementary-billing/format-sroc-transaction-line.service.js
index a9864d6ca6..0bb60a41b6 100644
--- a/app/services/supplementary-billing/format-sroc-transaction-line.service.js
+++ b/app/services/supplementary-billing/format-sroc-transaction-line.service.js
@@ -8,6 +8,7 @@
 
 const AbstractionBillingPeriodService = require('./abstraction-billing-period.service.js')
 const ConsolidateDateRangesService = require('./consolidate-date-ranges.service.js')
+const DetermineChargePeriodService = require('./determine-charge-period.service.js')
 
 /**
  * Takes a charge element, charge version and financial year and returns an object representing an sroc transaction
@@ -27,7 +28,7 @@ const ConsolidateDateRangesService = require('./consolidate-date-ranges.service.
 function go (chargeElement, chargeVersion, financialYearEnding, options) {
   const optionsData = _optionsDefaults(options)
 
-  const chargePeriod = _determineChargePeriod(chargeVersion, financialYearEnding)
+  const chargePeriod = DetermineChargePeriodService.go(chargeVersion, financialYearEnding)
 
   return {
     chargeElementId: chargeElement.chargeElementId,
@@ -78,31 +79,6 @@ function _optionsDefaults (options) {
   }
 }
 
-/**
- * Returns a charge period, which is an object comprising `startDate` and `endDate`
- *
- * The charge period is determined as the overlap between the charge version's start and end dates, and the financial
- * year. So the charge period's start date is the later of the two's start dates, and the charge period end date is the
- * earlier of the two's end dates.
- *
- * Note that charge versions may not have an end date; in this case, we simply use the financial year end date.
- */
-function _determineChargePeriod (chargeVersion, financialYearEnding) {
-  const financialYearStartDate = new Date(financialYearEnding - 1, 3, 1)
-  const chargeVersionStartDate = chargeVersion.startDate
-  const latestStartDateTimestamp = Math.max(financialYearStartDate, chargeVersionStartDate)
-
-  const financialYearEndDate = new Date(financialYearEnding, 2, 31)
-  // If the charge version has no end date then use the financial year end date instead
-  const chargeVersionEndDate = chargeVersion.endDate || financialYearEndDate
-  const earliestEndDateTimestamp = Math.min(financialYearEndDate, chargeVersionEndDate)
-
-  return {
-    startDate: new Date(latestStartDateTimestamp),
-    endDate: new Date(earliestEndDateTimestamp)
-  }
-}
-
 /**
  * Calculates the number of authorised days, ie. the number of overlapping days of the financial year and the charge
  * element's abstraction periods
diff --git a/test/services/supplementary-billing/determine-charge-period.service.test.js b/test/services/supplementary-billing/determine-charge-period.service.test.js
new file mode 100644
index 0000000000..62921d0575
--- /dev/null
+++ b/test/services/supplementary-billing/determine-charge-period.service.test.js
@@ -0,0 +1,172 @@
+'use strict'
+
+// Test framework dependencies
+const Lab = require('@hapi/lab')
+const Code = require('@hapi/code')
+
+const { describe, it, beforeEach } = exports.lab = Lab.script()
+const { expect } = Code
+
+// Thing under test
+const DetermineChargePeriodService = require('../../../app/services/supplementary-billing/determine-charge-period.service.js')
+
+describe('Determine charge period service', () => {
+  const financialYear = {
+    startDate: new Date('2022-04-01'),
+    endDate: new Date('2023-03-31'),
+    yearEnding: 2023
+  }
+  let chargeVersion
+
+  describe('charge version starts inside the financial period', () => {
+    beforeEach(() => {
+      chargeVersion = {
+        startDate: new Date('2022-05-01'),
+        endDate: null
+      }
+    })
+
+    describe('and the charge version has an end date', () => {
+      beforeEach(() => {
+        chargeVersion.endDate = new Date('2022-05-31')
+      })
+
+      it('returns the charge version start and end dates', () => {
+        const result = DetermineChargePeriodService.go(chargeVersion, financialYear.yearEnding)
+
+        expect(result.startDate).to.equal(chargeVersion.startDate)
+        expect(result.endDate).to.equal(chargeVersion.endDate)
+      })
+    })
+
+    describe('and the charge version does not have an end date', () => {
+      it('returns the charge version start date and financial year end date', () => {
+        const result = DetermineChargePeriodService.go(chargeVersion, financialYear.yearEnding)
+
+        expect(result.startDate).to.equal(chargeVersion.startDate)
+        expect(result.endDate).to.equal(financialYear.endDate)
+      })
+    })
+  })
+
+  describe('financial period starts inside the charge version', () => {
+    beforeEach(() => {
+      chargeVersion = {
+        startDate: new Date('2022-02-01'),
+        endDate: null
+      }
+    })
+
+    describe('and the charge version has an end date', () => {
+      beforeEach(() => {
+        chargeVersion.endDate = new Date('2023-05-31')
+      })
+
+      it('returns the financial start and end dates', () => {
+        const result = DetermineChargePeriodService.go(chargeVersion, financialYear.yearEnding)
+
+        expect(result.startDate).to.equal(financialYear.startDate)
+        expect(result.endDate).to.equal(financialYear.endDate)
+      })
+    })
+
+    describe('and the charge version does not have an end date', () => {
+      it('returns the financial start and end dates', () => {
+        const result = DetermineChargePeriodService.go(chargeVersion, financialYear.yearEnding)
+
+        expect(result.startDate).to.equal(financialYear.startDate)
+        expect(result.endDate).to.equal(financialYear.endDate)
+      })
+    })
+  })
+
+  describe('charge version starts before the financial period', () => {
+    beforeEach(() => {
+      chargeVersion = {
+        startDate: new Date('2022-02-01'),
+        endDate: null
+      }
+    })
+
+    describe('and the charge version has an end date that is inside the financial period', () => {
+      beforeEach(() => {
+        chargeVersion.endDate = new Date('2022-05-31')
+      })
+
+      it('returns the financial start and charge version end date', () => {
+        const result = DetermineChargePeriodService.go(chargeVersion, financialYear.yearEnding)
+
+        expect(result.startDate).to.equal(financialYear.startDate)
+        expect(result.endDate).to.equal(chargeVersion.endDate)
+      })
+    })
+
+    describe('and the charge version does not have an end date', () => {
+      it('returns the financial start and end dates', () => {
+        const result = DetermineChargePeriodService.go(chargeVersion, financialYear.yearEnding)
+
+        expect(result.startDate).to.equal(financialYear.startDate)
+        expect(result.endDate).to.equal(financialYear.endDate)
+      })
+    })
+  })
+
+  describe('financial period starts before the charge version', () => {
+    beforeEach(() => {
+      chargeVersion = {
+        startDate: new Date('2022-05-01'),
+        endDate: null
+      }
+    })
+
+    describe('and the charge version has an end date that is outside the financial period', () => {
+      beforeEach(() => {
+        chargeVersion.endDate = new Date('2023-05-31')
+      })
+
+      it('returns the charge version start date and financial period end date', () => {
+        const result = DetermineChargePeriodService.go(chargeVersion, financialYear.yearEnding)
+
+        expect(result.startDate).to.equal(chargeVersion.startDate)
+        expect(result.endDate).to.equal(financialYear.endDate)
+      })
+    })
+
+    describe('and the charge version does not have an end date', () => {
+      it('returns the charge version start date and financial period end date', () => {
+        const result = DetermineChargePeriodService.go(chargeVersion, financialYear.yearEnding)
+
+        expect(result.startDate).to.equal(chargeVersion.startDate)
+        expect(result.endDate).to.equal(financialYear.endDate)
+      })
+    })
+  })
+
+  describe('neither period overlaps', () => {
+    describe('because the charge version start date is after the financial period', () => {
+      beforeEach(() => {
+        chargeVersion = {
+          startDate: new Date('2023-05-01'),
+          endDate: null
+        }
+      })
+
+      it('throws an error', () => {
+        expect(() => DetermineChargePeriodService.go(chargeVersion, financialYear.yearEnding)).to.throw()
+      })
+    })
+
+    describe('because the charge version end date is before the financial period', () => {
+      beforeEach(() => {
+        chargeVersion = {
+          startDate: new Date('2021-05-01'),
+          endDate: new Date('2021-05-31')
+        }
+      })
+
+      it('throws an error', () => {
+        expect(() => DetermineChargePeriodService.go(chargeVersion, financialYear.yearEnding)).to.throw()
+      })
+    })
+  })
+})