diff --git a/get-cycle-day-number.js b/get-cycle-day-number.js
new file mode 100644
index 0000000..ff07b0c
--- /dev/null
+++ b/get-cycle-day-number.js
@@ -0,0 +1,46 @@
+export default function (cycleDays, targetDay) {
+  const lastFirstBleedingday = findLastFirstBleedingDay(cycleDays)
+
+  if (!lastFirstBleedingday) return
+
+  const diffInDays = targetDay.diff(lastFirstBleedingday.date, 'days')
+
+  // cycle starts at day 1
+  return diffInDays + 1
+}
+
+function findLastFirstBleedingDay(cycleDays) {
+  // sort the cycle days in descending order
+  // so we travel into the past as we iterate
+  // over the array
+  cycleDays.sort((a,b) => b.date - a.date)
+
+  let sawBleeding = false
+
+  for (let i = 0; i < cycleDays.length; i++) {
+    const cycleDay = cycleDays[i]
+
+    // we have detected the day before the beginning
+    // of a bleeding period, so the previous cycle day in the array
+    // was the first day of bleeding
+    if (sawBleeding && (!isBleedingDay(cycleDay))) {
+      return cycleDays[i - 1]
+    }
+
+    // if we get to the earliest cycle day and there was
+    // bleeding on that day, it's the first bleeding day for us
+    if (i === cycleDays.length - 1 && isBleedingDay(cycleDay)) {
+      return cycleDay
+    }
+
+    if (isBleedingDay(cycleDay)) {
+      sawBleeding = true
+    }
+  }
+}
+
+function isBleedingDay(cycleDay) {
+  return cycleDay.bleeding
+    && cycleDay.bleeding.value
+    && !cycleDay.exclude
+}
\ No newline at end of file
diff --git a/test/app.spec.js b/test/app.spec.js
deleted file mode 100644
index 019e3fa..0000000
--- a/test/app.spec.js
+++ /dev/null
@@ -1,5 +0,0 @@
-import { expect } from 'chai'
-
-it('says hello world', function () {
-  expect(1 + 5).to.eql(6)
-})
diff --git a/test/get-cycle-day.spec.js b/test/get-cycle-day.spec.js
new file mode 100644
index 0000000..a5572fd
--- /dev/null
+++ b/test/get-cycle-day.spec.js
@@ -0,0 +1,136 @@
+import { expect } from 'chai'
+import moment from 'moment'
+
+import getCycleDayNumber from '../get-cycle-day-number'
+
+describe('getCycleDay returns the cycle day', () => {
+  it('if the last data entered is a bleeding day', function () {
+    const cycleDays = [{
+      date: moment([2018, 5, 2])
+    }, {
+      date: moment([2018, 5, 3]),
+      bleeding: {
+        value: 2
+      }
+    }, {
+      date: moment([2018, 5, 4])
+    }, {
+      date: moment([2018, 5, 9]),
+      bleeding: {
+        value: 2
+      }
+    }, {
+      date: moment([2018, 5, 10]),
+      bleeding: {
+        value: 2
+      }
+    }]
+    const targetDate = moment([2018, 5, 17])
+    const result = getCycleDayNumber(cycleDays, targetDate)
+    expect(result).to.eql(9)
+  })
+
+  it('if the last data entered is a non-bleeding day', function () {
+    const cycleDays = [{
+      date: moment([2018, 5, 2])
+    }, {
+      date: moment([2018, 5, 3]),
+      bleeding: {
+        value: 2
+      }
+    }, {
+      date: moment([2018, 5, 4])
+    }, {
+      date: moment([2018, 5, 9]),
+      bleeding: {
+        value: 2
+      }
+    }, {
+      date: moment([2018, 5, 10]),
+      bleeding: {
+        value: 2
+      }
+    }, {
+      date: moment([2018, 5, 13])
+    }, {
+      date: moment([2018, 5, 14])
+    }]
+
+    const targetDate = moment([2018, 5, 17])
+    const result = getCycleDayNumber(cycleDays, targetDate)
+    expect(result).to.eql(9)
+  })
+
+  it('works if the cycle days are not sorted by date', function () {
+    const cycleDays = [{
+      date: moment([2018, 5, 13]),
+      bleeding: {
+        value: 2
+      }
+    }, {
+      date: moment([2018, 5, 9]),
+      bleeding: {
+        value: 2
+      }
+    }, {
+      date: moment([2018, 5, 3]),
+      bleeding: {
+        value: 2
+      }
+    }, {
+      date: moment([2018, 5, 4])
+    }, {
+      date: moment([2018, 5, 10]),
+      bleeding: {
+        value: 2
+      }
+    }, {
+      date: moment([2018, 5, 2])
+    }]
+
+    const targetDate = moment([2018, 5, 17])
+    const result = getCycleDayNumber(cycleDays, targetDate)
+    expect(result).to.eql(9)
+  })
+
+  it('if there are only bleeding days', function () {
+    const cycleDays = [{
+      date: moment([2018, 5, 9]),
+      bleeding: {
+        value: 2
+      }
+    }, {
+      date: moment([2018, 5, 10]),
+      bleeding: {
+        value: 2
+      }
+    }]
+    const targetDate = moment([2018, 5, 17])
+    const result = getCycleDayNumber(cycleDays, targetDate)
+    expect(result).to.eql(9)
+  })
+})
+
+describe('getCycleDay returns undefined', () => {
+  it('if there are no bleeding days', function () {
+    const cycleDays = [{
+      date: moment([2018, 5, 2])
+    }, {
+      date: moment([2018, 5, 4])
+    }, {
+      date: moment([2018, 5, 9]),
+    }, {
+      date: moment([2018, 5, 10]),
+    }]
+    const targetDate = moment([2018, 5, 17])
+    const result = getCycleDayNumber(cycleDays, targetDate)
+    expect(result).to.be.undefined
+  })
+
+  it('if there are no cycle days', function () {
+    const cycleDays = []
+    const targetDate = moment([2018, 5, 17])
+    const result = getCycleDayNumber(cycleDays, targetDate)
+    expect(result).to.be.undefined
+  })
+})
\ No newline at end of file