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diff --git a/date-fns/src/differenceInMilliseconds/test.ts b/date-fns/src/differenceInMilliseconds/test.ts
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+++ b/date-fns/src/differenceInMilliseconds/test.ts
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+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import differenceInMilliseconds from '.'
+
+describe('differenceInMilliseconds', function () {
+  it('returns the number of milliseconds between the given dates', function () {
+    const result = differenceInMilliseconds(
+      new Date(2014, 6 /* Jul */, 2, 12, 30, 20, 700),
+      new Date(2014, 6 /* Jul */, 2, 12, 30, 20, 600)
+    )
+    assert(result === 100)
+  })
+
+  it('returns a negative number if the time value of the first date is smaller', function () {
+    const result = differenceInMilliseconds(
+      new Date(2014, 6 /* Jul */, 2, 12, 30, 20, 600),
+      new Date(2014, 6 /* Jul */, 2, 12, 30, 20, 700)
+    )
+    assert(result === -100)
+  })
+
+  it('accepts timestamps', function () {
+    const result = differenceInMilliseconds(
+      new Date(2014, 8 /* Sep */, 5, 18, 30, 45, 500).getTime(),
+      new Date(2014, 8 /* Sep */, 5, 18, 30, 45, 500).getTime()
+    )
+    assert(result === 0)
+  })
+
+  it('does not return -0 when the given dates are the same', () => {
+    function isNegativeZero(x: number): boolean {
+      return x === 0 && 1 / x < 0
+    }
+
+    const result = differenceInMilliseconds(
+      new Date(2014, 8 /* Sep */, 5, 0, 0),
+      new Date(2014, 8 /* Sep */, 5, 0, 0)
+    )
+
+    const resultIsNegative = isNegativeZero(result)
+    assert(resultIsNegative === false)
+  })
+
+  it('returns NaN if the first date is `Invalid Date`', function () {
+    const result = differenceInMilliseconds(
+      new Date(NaN),
+      new Date(2017, 0 /* Jan */, 1)
+    )
+    assert(isNaN(result))
+  })
+
+  it('returns NaN if the second date is `Invalid Date`', function () {
+    const result = differenceInMilliseconds(
+      new Date(2017, 0 /* Jan */, 1),
+      new Date(NaN)
+    )
+    assert(isNaN(result))
+  })
+
+  it('returns NaN if the both dates are `Invalid Date`', function () {
+    const result = differenceInMilliseconds(new Date(NaN), new Date(NaN))
+    assert(isNaN(result))
+  })
+
+  it('throws TypeError exception if passed less than 2 arguments', function () {
+    assert.throws(differenceInMilliseconds.bind(null), TypeError)
+    assert.throws(differenceInMilliseconds.bind(null, 1), TypeError)
+  })
+})