Excel - How to convert the combination of year, week number and day of week to calendar day?

Question

Have >80000 records of the type:

Year Week Weekday
1971 40 1
1998 25 7
2020 34 3

In Excel, I want to convert the data (year: 1971-2020, week: 24-44, weekday: 1-7) to the calendar day (1-365; total number of days per year varying from year to year). Any suggestions? I find related posts, but none that solves my problem.

Answer 1

Leaving aside the regional specifics of what constitutes week 1 in a year where January 1 is not on the same day as the perceived start of the week, you can arrive at a running day number for each date by multiplying the week by 7 and adding the day.

= (Week * 7 ) + Weekday

Apply your regional settings of a Monday vs Sunday week start by adding or subtracting one day.

You can then even come up with a date using that number.

=date(Year,1, (Week * 7 ) + Weekday)

This will use the first day of January as the first week, though. If that doesn't suit, you'll need to adjust by working out what week 1 means in any one year.

Edit:

Here is a table with your sample data:

The formula for year day is

=[@Week]*7 

The formula for adjust year start is

=WEEKDAY(DATE([@Year],1,1))

The formula for the final date is

=DATE([@Year],1,[@[year day]])-[@[adjust year start]]

You can see that year 2020, week 53, day 5 is shown as 1st January 2021, so that matches up. You can move the adjust year start calculation into the year day column, of course.