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diff --git a/date-fns/src/differenceInHours/test.ts b/date-fns/src/differenceInHours/test.ts
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+// @flow
+/* eslint-env mocha */
+
+import assert from 'assert'
+import differenceInHours from '.'
+
+describe('differenceInHours', function() {
+  it('returns the number of hours between the given dates', function() {
+    const result = differenceInHours(
+      new Date(2014, 6 /* Jul */, 2, 20, 0),
+      new Date(2014, 6 /* Jul */, 2, 6, 0)
+    )
+    assert(result === 14)
+  })
+
+  it('returns a negative number if the time value of the first date is smaller', function() {
+    const result = differenceInHours(
+      new Date(2014, 6 /* Jul */, 2, 6, 0),
+      new Date(2014, 6 /* Jul */, 2, 20, 0)
+    )
+    assert(result === -14)
+  })
+
+  it('accepts timestamps', function() {
+    const result = differenceInHours(
+      new Date(2014, 8 /* Sep */, 5, 18, 0).getTime(),
+      new Date(2014, 8 /* Sep */, 5, 6, 0).getTime()
+    )
+    assert(result === 12)
+  })
+
+  describe('edge cases', function() {
+    it('the difference is less than an hour, but the given dates are in different calendar hours', function() {
+      const result = differenceInHours(
+        new Date(2014, 8 /* Sep */, 5, 12),
+        new Date(2014, 8 /* Sep */, 5, 11, 59)
+      )
+      assert(result === 0)
+    })
+
+    it('the same for the swapped dates', function() {
+      const result = differenceInHours(
+        new Date(2014, 8 /* Sep */, 5, 11, 59),
+        new Date(2014, 8 /* Sep */, 5, 12)
+      )
+      assert(result === 0)
+    })
+
+    it('the difference is an integral number of hours', function() {
+      const result = differenceInHours(
+        new Date(2014, 8 /* Sep */, 5, 13, 0),
+        new Date(2014, 8 /* Sep */, 5, 12, 0)
+      )
+      assert(result === 1)
+    })
+
+    it('the given dates are the same', function() {
+      const result = differenceInHours(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+      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 = differenceInHours(
+        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 = differenceInHours(
+      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 = differenceInHours(
+      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 = differenceInHours(new Date(NaN), new Date(NaN))
+    assert(isNaN(result))
+  })
+
+  it('throws TypeError exception if passed less than 2 arguments', function() {
+    //@ts-expect-error
+    assert.throws(differenceInHours.bind(null), TypeError)
+    //@ts-expect-error
+    assert.throws(differenceInHours.bind(null, 1), TypeError)
+  })
+})