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diff --git a/date-fns/src/formatISODuration/test.js b/date-fns/src/formatISODuration/test.js
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+++ b/date-fns/src/formatISODuration/test.js
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+// @flow
+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import formatISODuration from '.'
+import intervalToDuration from '../intervalToDuration'
+
+describe('formatISODuration', function() {
+  it('Everything returns correct duration for arbitrary dates', function() {
+    const start = new Date(1929, 0, 15, 12, 0, 0)
+    const end = new Date(1968, 3, 4, 19, 5, 0)
+    const result = formatISODuration(intervalToDuration({ start, end }))
+
+    assert(result === 'P39Y2M20DT7H5M0S')
+  })
+  it('Everything returns P1Y1M1DT1H1M1S (1 of everything)', function() {
+    const start = new Date(2020, 2, 1, 12, 0, 0)
+    const end = new Date(2021, 3, 2, 13, 1, 1)
+    const result = formatISODuration(intervalToDuration({ start, end }))
+
+    assert(result === 'P1Y1M1DT1H1M1S')
+  })
+  it('Returns P0Y0M0DT0H0M0S when the dates are the same', function() {
+    const start = new Date(2020, 2, 1, 12, 0, 0)
+    const end = new Date(2020, 2, 1, 12, 0, 0)
+    const result = formatISODuration(intervalToDuration({ start, end }))
+
+    assert(result === 'P0Y0M0DT0H0M0S')
+  })
+  it('Seconds returns P0Y0M0DT0H0M1S (1 second)', function() {
+    const start = new Date(2020, 2, 1, 12, 0, 0)
+    const end = new Date(2020, 2, 1, 12, 0, 1)
+    const result = formatISODuration(intervalToDuration({ start, end }))
+
+    assert(result === 'P0Y0M0DT0H0M1S')
+  })
+  it('Minutes returns P0Y0M0DT0H1M0S (1 minute)', function() {
+    const start = new Date(2020, 2, 1, 12, 0, 0)
+    const end = new Date(2020, 2, 1, 12, 1, 0)
+    const result = formatISODuration(intervalToDuration({ start, end }))
+
+    assert(result === 'P0Y0M0DT0H1M0S')
+  })
+  it('Hours returns P0Y0M0DT1H0M0S (1 hour)', function() {
+    const start = new Date(2020, 2, 1, 12, 0, 0)
+    const end = new Date(2020, 2, 1, 13, 0, 0)
+    const result = formatISODuration(intervalToDuration({ start, end }))
+
+    assert(result === 'P0Y0M0DT1H0M0S')
+  })
+  it('Days returns P0Y0M1DT0H0M0S (1 day)', function() {
+    const start = new Date(2020, 2, 1, 12, 0, 0)
+    const end = new Date(2020, 2, 2, 12, 0, 0)
+    const result = formatISODuration(intervalToDuration({ start, end }))
+
+    assert(result === 'P0Y0M1DT0H0M0S')
+  })
+  it('Months returns P0Y1M0DT0H0M0S (1 month)', function() {
+    const start = new Date(2020, 2, 1, 12, 0, 0)
+    const end = new Date(2020, 3, 1, 12, 0, 0)
+    const result = formatISODuration(intervalToDuration({ start, end }))
+
+    assert(result === 'P0Y1M0DT0H0M0S')
+  })
+  it('Years returns P1Y0M0DT0H0M1S (1 year)', function() {
+    const start = new Date(2020, 2, 1, 12, 0, 0)
+    const end = new Date(2021, 2, 1, 12, 0, 0)
+    const result = formatISODuration(intervalToDuration({ start, end }))
+
+    assert(result === 'P1Y0M0DT0H0M0S')
+  })
+
+  it('returns P0Y0M0DT0H0M0S when given an empty object', function() {
+    const result = formatISODuration({})
+
+    assert(result === 'P0Y0M0DT0H0M0S')
+  })
+})