Generate an 8-day datetime array between 2010 and 2021 which resets the 8-day count at the beginning of each year

Hello, I need to generate an 8-day datetime array between 2010 and 2021; however, i need the 8-day interval to reset at the beginning of each year. For example, the final date in 2010 would be December 27 and i would like the next date to be January 1 2011, not January 4. The code below gives the 8-day intervals without resetting.

t1 = datetime(2010,1,1,0,0,0);
t2 = datetime(2021,12,31,0,0,0);
t = t1:caldays(8):t2;

Answer 1

[Steven Lord]

Let's start with a row vector of January 1sts.

Jan1st = datetime(2010:2021, 1, 1);

We want to compute a vector of days for each January 1st that increases by 8 days until the end of the year. Luckily the number of days in the year is not a multiple of 8, so each year will have the same number of values represented.

dayIncrements = days(0:8:365).';

If you were using release R2020b or later you could use implicit expansion to sum Jan1st and dayIncrements to generate a matrix. If you need this as a vector you could reshape it. Since you indicated you're using release R2018a you'd need to repmat the two vectors.

t = Jan1st + dayIncrements;
t2 = repmat(Jan1st, numel(dayIncrements), 1) + repmat(dayIncrements, 1, numel(Jan1st));
check = isequal(t, t2)

check = logical
   1

Now let's look at the first and last days in each column. Leap years end on December 26th, non leap years on December 27th.

firstAndLast = t([1 end], :)

firstAndLast = 2×12 datetime array
   01-Jan-2010   01-Jan-2011   01-Jan-2012   01-Jan-2013   01-Jan-2014   01-Jan-2015   01-Jan-2016   01-Jan-2017   01-Jan-2018   01-Jan-2019   01-Jan-2020   01-Jan-2021
   27-Dec-2010   27-Dec-2011   26-Dec-2012   27-Dec-2013   27-Dec-2014   27-Dec-2015   26-Dec-2016   27-Dec-2017   27-Dec-2018   27-Dec-2019   26-Dec-2020   27-Dec-2021

[Ziad Sari El Dine]

Thanks!