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diff --git a/public/javascripts/getWeek.js b/public/javascripts/getWeek.js
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+// copied from http://www.meetingpointmco.nl/Roosters-AL/doc/dagroosters/untisscripts.js,
+// were using the same code as they do to be sure that we always get the same
+// week number.
+function getWeek () {
+  // Create a copy of this date object
+  var target = new Date()
+
+  // ISO week date weeks start on monday
+  // so correct the day number
+  var dayNr = (target.getDay() + 6) % 7
+
+  // ISO 8601 states that week 1 is the week
+  // with the first thursday of that year.
+  // Set the target date to the thursday in the target week
+  target.setDate(target.getDate() - dayNr + 3)
+
+  // Store the millisecond value of the target date
+  var firstThursday = target.valueOf()
+
+  // Set the target to the first thursday of the year
+  // First set the target to january first
+  target.setMonth(0, 1)
+  // Not a thursday? Correct the date to the next thursday
+  if (target.getDay() !== 4) {
+    target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7)
+  }
+
+  // The weeknumber is the number of weeks between the
+  // first thursday of the year and the thursday in the target week
+  return 1 + Math.ceil((firstThursday - target) / 604800000) // 604800000 = 7 * 24 * 3600 * 1000
+}
+
+module.exports = getWeek