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diff --git a/date-fns/src/subMonths/test.ts b/date-fns/src/subMonths/test.ts
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+++ b/date-fns/src/subMonths/test.ts
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+// @flow
+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import subMonths from '.'
+
+describe('subMonths', function() {
+  it('subtracts the given number of months', function() {
+    const result = subMonths(new Date(2015, 1 /* Feb */, 1), 5)
+    assert.deepEqual(result, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('accepts a timestamp', function() {
+    const result = subMonths(new Date(2015, 8 /* Sep */, 1).getTime(), 12)
+    assert.deepEqual(result, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('converts a fractional number to an integer', function() {
+    const result = subMonths(new Date(2015, 1 /* Feb */, 1), 5.999)
+    assert.deepEqual(result, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('implicitly converts number arguments', function() {
+    // $ExpectedMistake
+    // @ts-expect-error
+    const result = subMonths(new Date(2015, 1 /* Feb */, 1), '5')
+    assert.deepEqual(result, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('does not mutate the original date', function() {
+    const date = new Date(2014, 8 /* Sep */, 1)
+    subMonths(date, 12)
+    assert.deepEqual(date, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('works well if the desired month has fewer days and the provided date is in the last day of a month', function() {
+    const date = new Date(2014, 11 /* Dec */, 31)
+    const result = subMonths(date, 3)
+    assert.deepEqual(result, new Date(2014, 8 /* Sep */, 30))
+  })
+
+  it('handles dates before 100 AD', function() {
+    const initialDate = new Date(0)
+    initialDate.setFullYear(1, 2 /* Mar */, 31)
+    initialDate.setHours(0, 0, 0, 0)
+    const expectedResult = new Date(0)
+    expectedResult.setFullYear(1, 1 /* Feb */, 28)
+    expectedResult.setHours(0, 0, 0, 0)
+    const result = subMonths(initialDate, 1)
+    assert.deepEqual(result, expectedResult)
+  })
+
+  it('returns `Invalid Date` if the given date is invalid', function() {
+    const result = subMonths(new Date(NaN), 5)
+    // @ts-expect-error
+    assert(result instanceof Date && isNaN(result))
+  })
+
+  it('returns `Invalid Date` if the given amount is NaN', function() {
+    const result = subMonths(new Date(2015, 1 /* Feb */, 1), NaN)
+    // @ts-expect-error
+    assert(result instanceof Date && isNaN(result))
+  })
+
+  it('throws TypeError exception if passed less than 2 arguments', function() {
+    assert.throws(subMonths.bind(null), TypeError)
+    assert.throws(subMonths.bind(null, 1), TypeError)
+  })
+})