Add days to a specific date in datetime2 in a for loop / foreach loop

Question

I want to calculate the beginning and ending date of a week (first day is Monday, last is Sunday). This is working fine for a single date. Now I want to calculate the start and end of multiple weeks consecutively in a loop, given the starting date (always a Monday) and the number of weeks to calculate. This works only for the first calculation, meaning that the date of the next Sunday is correct, but after that the date is always reset after each loop.

My algorithm to calculate the next date looks like this:

# cursor is at a Monday
for n in 1 .. 10:
    print(cursor) # Monday
    add 6 days to cursor
    print(cursor) # Sunday
    add 1 day to cursor
end for

Minimum Working Example:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[calc]{datetime2}
\usepackage{pgffor}
\setlength{\parindent}{0pt}
\begin{document}
\newcommand{\nweeks}{4}
\newcommand{\startdate}{2020-09-07}
\newcount\datecount
\newcommand{\dateoffset}[2]{%
    \DTMsaveddateoffsettojulianday{#1}{#2}{\datecount}%
    \DTMsavejulianday{#1}{\datecount}%
}
\DTMsavedate{currentdate}{\startdate}
Start-Week-1: \DTMusedate{currentdate}\
\dateoffset{currentdate}{6}
End-Week-1: \DTMusedate{currentdate}\
\dateoffset{currentdate}{1}
Start-Week-2: \DTMusedate{currentdate}\
\dateoffset{currentdate}{6}
End-Week-2: \DTMusedate{currentdate}\
\dateoffset{currentdate}{1}
\DTMsavedate{currentdate}{\startdate}
\foreach \n in {1,...,\nweeks}{
    Start-Week-\n: \DTMusedate{currentdate}\
    \dateoffset{currentdate}{6}
    End-Week-\n: \DTMusedate{currentdate}\
    \dateoffset{currentdate}{1}
}
\end{document}

Answer 1

I'm afraid you've fallen into the typical trap of pgffor's \foreach macro: it evaluates the loop body inside a TeX group, therefore local assignements performed therein are lost at the end of each iteration. My suggestion would be to use expl3's \int_step_inline:nnn macro or one of its sisters. Another option would be to perform a global assignment to the \datecount count register at the end of the loop; however, mixing local and global assignments to the same macro or register should be avoided in general (cf. chapter 27 of the TeXbook regarding “save stack buildup”). Here is a solution using expl3's \int_step_inline:nnn macro:

\documentclass{article}
\usepackage{expl3}
\usepackage[calc]{datetime2}
\ExplSyntaxOn
% Borrow \int_step_inline:nnn from expl3
\cs_new_eq:NN \intstepinline \int_step_inline:nnn
\ExplSyntaxOff
\newcount\datecount
\newcommand*{\dateoffset}[2]{%
    \DTMsaveddateoffsettojulianday{#1}{#2}{\datecount}%
    \DTMsavejulianday{#1}{\datecount}%
}
\newcommand{\nweeks}{4}
\newcommand{\startdate}{2020-09-07}
\DTMsavedate{currentdate}{\startdate}
\setlength{\parindent}{0pt}
\begin{document}
\intstepinline{1}{\nweeks}{%
  Start-Week-#1: \DTMusedate{currentdate}%
  \dateoffset{currentdate}{6}\
  End-Week-#1: \DTMusedate{currentdate}%
  \dateoffset{currentdate}{1}%
  \par
}
\end{document}