## Explanation

This formula builds the final result in 2 parts, joined by concatenation with the ampersand (&) operator.

On the left of the ampersand, we generate the year value. To extract a 2-digit year, we can use the TEXT function, which can apply a number format inside a formula:

```
TEXT(B5,"yy")
```

To extract a full year, use the YEAR function:

```
YEAR(B5)
```

On the right side of the ampersand we need to figure out the day of year. We do this by subtracting the last day of the previous year from the date we are working with. Because dates are just serial numbers, this will give us the "nth" day of year.

To get the last day of year of the previous year, we use the DATE function. When you give DATE a year and month value, and a zero for day, you get the last day of the previous month. So:

```
B5-DATE(YEAR(B5),1,0)
```

gives us the last day of the previous year, which is December 31, 2015 in the example.

Now we need to pad the day value with zeros. Again, we can use the TEXT function:

```
TEXT(B5-DATE(YEAR(B5),1,0),"000")
```

### Reverse Julian date

If you need to convert a Julian date back to a regular date, you can use a formula that parses Julian date and runs it through the date function with a month of 1 and day equal to the "nth" day. For example, this would create a date from a yyyyddd Julian date like 1999143.

```
=DATE(LEFT(A1,4),1,RIGHT(A1,3)) // for yyyyddd
```

If you just have a day number (e.g. 100, 153, etc.), you can hard-code the year and insert the day this:

```
=DATE(2016,1,A1)
```

Where A1 contains the day number. This works because the DATE function knows how to adjust for values that are out of range.