Add dayOfYear

snippets/dayOfYear.md

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+### dayOfYear
+
+Gets the day of the year from a `Date` object.
+
+Use `new Date()` and `Date.prototype.getFullYear()` to get the first day of the year as a `Date` object, subtract it from the provided `date` and divide with the milliseconds in each day to get the result.
+Use `Math.floor()` to appropriately round the resulting day count to an integer.
+
+```js
+const dayOfYear = date => Math.floor(
+  (date - new Date(date.getFullYear(), 0, 0)) / 1000 / 60 / 60 / 24
+);
+```
+
+```js
+dayOfYear(new Date()); // 272
+```