خودکار میشه اینکار رو کرد اما به زحمتش نمیارزه. برای این کار اول باید 10 روز به دستور بدوی \day اضافه کنی و بعد تمام محاسباتی رو که بسته xepersian-persiancal انجام میده، دوباره انجام بدی.
مثلاً ماکروی \futuredate رو اینجوری تعریف کنی:
\makeatletter
\newcommand*{\futuredate}[1]{%
\begingroup%
\advance \day by #1%
\XePersian@y=\year \XePersian@m=\month \XePersian@d=\day
\XePersian@temp=\XePersian@y
\divide\XePersian@temp by 100\relax
\multiply\XePersian@temp by 100\relax
\XePersian@yModHundred=\XePersian@y
\advance\XePersian@yModHundred by -\XePersian@temp\relax
\ifodd\XePersian@yModHundred
\XePersian@leapfalse
\else
\XePersian@temp=\XePersian@yModHundred
\divide\XePersian@temp by 2\relax
\ifodd\XePersian@temp\XePersian@leapfalse
\else
\ifnum\XePersian@yModHundred=0%
\XePersian@temp=\XePersian@y
\divide\XePersian@temp by 400\relax
\multiply\XePersian@temp by 400\relax
\ifnum\XePersian@y=\XePersian@temp\XePersian@leaptrue\else\XePersian@leapfalse\fi
\else\XePersian@leaptrue
\fi
\fi
\fi
\XePersian@latini=31\relax
\ifXePersian@leap
\XePersian@latinii = 29\relax
\else
\XePersian@latinii = 28\relax
\fi
\XePersian@latiniii = 31\relax
\XePersian@latiniv = 30\relax
\XePersian@latinv = 31\relax
\XePersian@latinvi = 30\relax
\XePersian@latinvii = 31\relax
\XePersian@latinviii = 31\relax
\XePersian@latinix = 30\relax
\XePersian@latinx = 31\relax
\XePersian@latinxi = 30\relax
\XePersian@latinxii = 31\relax
\XePersian@thirtytwo=32\relax
\XePersian@temp=\XePersian@y
\advance\XePersian@temp by -17\relax
\XePersian@temptwo=\XePersian@temp
\divide\XePersian@temptwo by 33\relax
\multiply\XePersian@temptwo by 33\relax
\advance\XePersian@temp by -\XePersian@temptwo
\ifnum\XePersian@temp=\XePersian@thirtytwo\XePersian@kabisehfalse
\else
\XePersian@temptwo=\XePersian@temp
\divide\XePersian@temptwo by 4\relax
\multiply\XePersian@temptwo by 4\relax
\advance\XePersian@temp by -\XePersian@temptwo
\ifnum\XePersian@temp=\z@\XePersian@kabisehtrue\else\XePersian@kabisehfalse\fi
\fi
\XePersian@tempthree=\XePersian@y % Number of Leap years
\advance\XePersian@tempthree by -1
\XePersian@temp=\XePersian@tempthree % T := (MY-1) div 4
\divide\XePersian@temp by 4\relax
\XePersian@temptwo=\XePersian@tempthree % T := T - ((MY-1) div 100)
\divide\XePersian@temptwo by 100\relax
\advance\XePersian@temp by -\XePersian@temptwo
\XePersian@temptwo=\XePersian@tempthree % T := T + ((MY-1) div 400)
\divide\XePersian@temptwo by 400\relax
\advance\XePersian@temp by \XePersian@temptwo
\advance\XePersian@tempthree by -611 % Number of Kabise years
\XePersian@temptwo=\XePersian@tempthree % T := T - ((SY+10) div 33) * 8
\divide\XePersian@temptwo by 33\relax
\multiply\XePersian@temptwo by 8\relax
\advance\XePersian@temp by -\XePersian@temptwo
\XePersian@temptwo=\XePersian@tempthree %
\divide\XePersian@temptwo by 33\relax
\multiply\XePersian@temptwo by 33\relax
\advance\XePersian@tempthree by -\XePersian@temptwo
\ifnum\XePersian@tempthree=32\advance\XePersian@temp by 1\fi % if (SY+10) mod 33=32 then Inc(T);
\divide\XePersian@tempthree by 4\relax % T := T - ((SY+10) mod 33) div 4
\advance\XePersian@temp by -\XePersian@tempthree
\advance\XePersian@temp by -137 % T := T - 137 Adjust the value
\XePersian@persiani=31
\advance\XePersian@persiani by -\XePersian@temp % now 31 - T is the persiani
\XePersian@persianii = 30\relax
\ifXePersian@kabiseh
\XePersian@persianiii = 30\relax
\else
\XePersian@persianiii = 29\relax
\fi
\XePersian@persianiv = 31\relax
\XePersian@persianv = 31\relax
\XePersian@persianvi = 31\relax
\XePersian@persianvii = 31\relax
\XePersian@persianviii= 31\relax
\XePersian@persianix = 31\relax
\XePersian@persianx = 30\relax
\XePersian@persianxi = 30\relax
\XePersian@persianxii = 30\relax
\XePersian@persianxiii= 30\relax
\XePersian@dn= 0\relax
\XePersian@sn= 0\relax
\XePersian@mminusone=\XePersian@m
\advance\XePersian@mminusone by -1\relax
\XePersian@i=0\relax
\ifnum\XePersian@i < \XePersian@mminusone
\loop
\advance \XePersian@i by 1\relax
\advance\XePersian@dn by \csname XePersian@latin\romannumeral\the\XePersian@i\endcsname
\ifnum\XePersian@i<\XePersian@mminusone \repeat
\fi
\advance \XePersian@dn by \XePersian@d
\XePersian@i=1\relax
\XePersian@sn = \XePersian@persiani
\ifnum \XePersian@sn<\XePersian@dn
\loop
\advance \XePersian@i by 1\relax
\advance\XePersian@sn by \csname XePersian@persian\romannumeral\the\XePersian@i\endcsname
\ifnum \XePersian@sn<\XePersian@dn \repeat
\fi
\ifnum \XePersian@i < 4
\XePersian@m = 9 \advance\XePersian@m by \XePersian@i
\advance \XePersian@y by -622\relax
\else
\XePersian@m = \XePersian@i \advance \XePersian@m by -3\relax
\advance \XePersian@y by -621\relax
\fi
\advance\XePersian@sn by -\csname XePersian@persian\romannumeral\the\XePersian@i%
\endcsname
\ifnum\XePersian@i = 1
\XePersian@d = \XePersian@dn \advance \XePersian@d by 30 \advance\XePersian@d by -\XePersian@persiani
\else
\XePersian@d = \XePersian@dn \advance \XePersian@d by -\XePersian@sn
\fi
\today
\endgroup}
\makeatother
دستور \futuredate یک آرگومان داره که تعداد روز رو جلوتر میبره. بنابراین
\futuredate{10}
تاریخ رو ۱۰ روز جلوتر میبره. اما این روش رو پیشنهاد نمیکنم چون هم طولانیه هم شما چیزی ازش نمیفهمید و اگر مشکلی پیش بیاد، نمیتونید حلش کنید.
بنابراین راحتتره که تاریخ ۱۰ روز دیگر را دستی بدی.