I have a dataframe that looks like this:

```
Year    vl
2017    20
2017    21
2017    22
2017    23
2017    24
2017    25
2017    26
...

```

I need to convert the year into the format dd.mm.yyyy. Every time start from the first day of the year. For example, 2017 will become 01.01.2017. And then, I need to multiply each value in the column "vl" by 7 and add them line by line to the column as the number of days, where the dates will be in the new format (as in the example 01.01.2017).
The result should be something like this:

```
Year    vl     new_date
2017    20    21.05.2017
2017    21    28.05.2017
2017    22    04.06.2017
2017    23    11.06.2017
2017    24    18.06.2017
2017    25    25.06.2017
2017    26    02.07.2017
...

```