I have made a command that looks like this:

newcommand*numdash,--,

ExplSyntaxOn
NewDocumentCommanddateRangemmmmmm
 
 str_case:nnF #1 
 
 #4 
 str_case:nnF #2 
 
 #5 
 str_case:nnF #3 
 
 #6 DTMdisplaydate#4#5#6-1 %
 DTMordinal#3numdashDTMdisplaydate#4#5#6-1
 
 DTMordinal#3~DTMmonthname#2%
 numdashDTMdisplaydate#4#5#6-1
 
 DTMdisplaydate#1#2#3-1numdashDTMdisplaydate#4#5#6-1
 
ExplSyntaxOff

It displays the duration from one date to the next in a succinct matter. See the image below.

However, I need to manipulate the end date by incrementing it by a few days.
Something like dateRange2016123120161231 + 1 does not work.
I found the following code

%https://tex.stackexchange.com/questions/318006/add-n-days-to-variable-date
DTMsavedateDeadLineDate2016-05-20
newcommandDeadLineDateExtend1

newcountdaycount
newcommanddueDate[1]%
 DTMsaveddateoffsettojuliandayDeadLineDate#1daycount
 DTMsavejuliandaynewDeadLineDatenumberdaycount
 DTMusedatenewDeadLineDate
 

Which can add days to a DTMdate. My problem is making these two functions work together.

1. Rewrite dateRange so that it inputs two DTMdates instead of the plain numbers.


2. Exctract the year, month and date from newDeadLineDate and insert it back into dateRange
   

So what I need help with is to first have two dates, increment one of them and display the daterange using dateRange function. Any help with 1. or 2. from the list above is greatly appreciated.

documentclassarticle

usepackage[english]babel
usepackage[en-GB,calc]datetime2

usepackagexparse

% https://tex.stackexchange.com/questions/390693/datetime-ranges-using-datetime2/390738
newcommand*numdash,--,

ExplSyntaxOn
NewDocumentCommanddateRangemmmmmm
 
 str_case:nnF #1 
 
 #4 
 str_case:nnF #2 
 
 #5 
 str_case:nnF #3 
 
 #6 DTMdisplaydate#4#5#6-1 %
 DTMordinal#3numdashDTMdisplaydate#4#5#6-1
 
 DTMordinal#3~DTMmonthname#2%
 numdashDTMdisplaydate#4#5#6-1
 
 DTMdisplaydate#1#2#3-1numdashDTMdisplaydate#4#5#6-1
 
ExplSyntaxOff

%https://tex.stackexchange.com/questions/318006/add-n-days-to-variable-date
DTMsavedateDeadLineDate2016-05-20
newcommandDeadLineDateExtend1

newcountdaycount
newcommanddueDate[1]%
 DTMsaveddateoffsettojuliandayDeadLineDate#1daycount
 DTMsavejuliandaynewDeadLineDatenumberdaycount
 DTMusedatenewDeadLineDate
 

begindocument

dateRange2016123120161231

dateRange2016123020161231

dateRange2016113120161231

dateRange2015123120161231

enddocument

Extracting the year, month and day from a named datetime2 date

As you probably noticed in your code sample where the commands DTMsavedate and DTMsavejulianday are used, the datetime2 package can save dates in special stores identified by a 〈name〉. Quoting section Storing and Using Dates and Times of the datetime2 manual:

In the commands below, the 〈name〉 (no active characters) is a name that uniquely identifies the information.

When you have a date in such a store, as is the case after executing DTMsavejulianday〈name〉〈number〉, you can extract the corresponding year, month and day using the commands DTMfetchyear, DTMfetchmonth and DTMfetchday. Each of these commands takes a 〈name〉 argument and expands to the corresponding number. In other words, DTMfetchyear〈name〉 expands to the year, DTMfetchmonth〈name〉 to the month and DTMfetchday〈name〉 to the day. The rest is essentially plumbing work. :-)

Connecting the different parts

I propose a dateRange command that behaves like yours, except that:

• it accepts an optional argument (defaulting to 0) that specifies the number of days you want to add to the end date before displaying the resulting range;




• it uses a no-break space between the day ordinal and the month name (~ is a normal space under ExplSyntaxOn regime, but I believe you wanted a no-break space);




• it uses integer comparisons, so that 01 is considered the same as 1, 02 the same as 2, etc. for all day, month and year arguments (if you don't want that, use str_if_eq:nnTF as in the first revision of this answer, but I don't see any good reason to do so).

I reindented the function and used int_compare:nNnTF instead of str_case:nnF to make the code easier to read and address the third point above.

I also defined a code-level function nebuch_display_date_range:nnnnnn to contain this reworked code, because it is much easier to reuse in different situations this way. This allows us to generate a variant nebuch_display_date_range:nnnxxx that fully expands the three last arguments before passing them to the base function nebuch_display_date_range:nnnnnn (full expansion as done by edef). This only requires a simple line of code:

cs_generate_variant:Nn nebuch_display_date_range:nnnnnn nnnxxx 

Using the variant, it's easy to pass the result of the temporary date calculation (which adds the offset) to the base function nebuch_display_date_range:nnnnnn, because DTMfetchday, DTMfetchmonth and DTMfetchyear are all expandable functions (see the documentation of datetime2).

The computation of the offset date is also implemented in a code-level function, namely nebuch_compute_offset_date:nnnnn, to make it easily reusable by other code (and it is used in my implementation of dateRange).

documentclassarticle
usepackage[english]babel
usepackage[en-GB,calc]datetime2
usepackagexparse

newcommand*numdash,--,

ExplSyntaxOn

% Same as your dateRange, but the code-level API allows the creation of
% variants with cs_generate_variant:Nn.
cs_new_protected:Npn nebuch_display_date_range:nnnnnn #1#2#3#4#5#6
 
 int_compare:nNnTF #1 = #4
 
 int_compare:nNnTF #2 = #5
 
 int_compare:nNnTF #3 = #6
 DTMdisplaydate #4 #5 #6 -1 
 
 DTMordinal #3 numdash
 DTMdisplaydate #4 #5 #6 -1
 
 
 
 % I replaced ~ with nobreakspace here because of ExplSyntaxOn
 DTMordinal #3 nobreakspace DTMmonthname #2 numdash
 DTMdisplaydate #4 #5 #6 -1
 
 
 
 DTMdisplaydate #1 #2 #3 -1 numdash
 DTMdisplaydate #4 #5 #6 -1
 
 

cs_generate_variant:Nn nebuch_display_date_range:nnnnnn nnnxxx 

newcount nebuch_tmp_count

% #1, #2, #3: year, month, day
% #4: number of days (offset)
% #5: <name> (in the sense of datetime2) used to store the resulting date
cs_new_protected:Npn nebuch_compute_offset_date:nnnnn #1#2#3#4#5
 
 DTMsavedate nebuch_compute_offset_date_tmp_date #1-#2-#3 
 DTMsaveddateoffsettojulianday
 nebuch_compute_offset_date_tmp_date #4 nebuch_tmp_count 
 DTMsavejulianday #5 number nebuch_tmp_count 
 

NewDocumentCommand dateRange O0 m m m m m m 
 
 nebuch_compute_offset_date:nnnnn #5 #6 #7 #1
 nebuch_dateRange_tmp_date 
 nebuch_display_date_range:nnnxxx #2 #3 #4
 DTMfetchyear nebuch_dateRange_tmp_date 
 DTMfetchmonth nebuch_dateRange_tmp_date 
 DTMfetchday nebuch_dateRange_tmp_date 
 

ExplSyntaxOff

begindocument

dateRange2016123120161231par
dateRange2016123020161231par
dateRange2016113120161231par
dateRange2015123120161231

bigskip
dateRange[3]2016123120161231par
dateRange[3]2016123120161228par
dateRange[3]2016123020161231par
dateRange[3]2016123020161228par
dateRange[31]2016113120161231par
dateRange[31]2016113120161130par
dateRange[365]2015123120161231par
dateRange[365]2015123120160101

bigskip
dateRange20160272016207

enddocument