yesterday

daysinmonth = {31,28,31,30,31,30,31,31,30,31,30,31};
intmonthnames = {"January"  , "February", "March"   , "April",
                 "May"      , "June"    , "July"    , "August",
                 "September", "October" , "November", "December"};
weekdaynames = {[0]="Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"};
currentyear = tonumber(os.date("%Y"));
currentmonth = tonumber(os.date("%m"));
currentday = tonumber(os.date("%d"));
currentweekday = tonumber(os.date("%w"));
function dateYesterday()
  local cd = currentday;
  local cm = currentmonth;
  local cwd = currentweekday;
  local cy = currentyear;
  local yd,ym,ywd,yy;  -- previous day, previous month, previous weekday, previous year
  if cd > 1 then-- if current day is greater than one
    yd = cd - 1; -- previous day equals current day minus one
    if cwd > 0 then -- if current week day greater than zero
      ywd = cwd - 1; -- previous weekday equals current weekday minus one
    else
      ywd = 6; -- previous weekday equals six
    end
  else -- if current day equals one, we have to go back to the previous month
    if cm > 1 then -- if current month greater than one
      ym = cm - 1; -- previous month equals current month minus one
      if ym == 2 then -- if previous month equals 2
        if ( cy % 4 ) == 0 then -- we would have a leap year.. ignoring 400 year leaps
          cd = 29;
        else
          cd = daysinmonth[ym];
        end
      end
    else  -- if current month equals one
      yy = cy - 1; -- previous year equals current year minus one
      ym = 12; -- previous month equals twelve
      yd = 31; -- december has 31 days so yesterday was the 31th
    end
  end
  return yd,ym or cm,ywd,yy or cy;
end

function prettyPrintDate(day,month,weekday,year)
  -- weekdaynames = {[0]="Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"};
  -- intmonthnames = {"January"  , "February", "March"   , "April",
  --                  "May"      , "June"    , "July"    , "August",
  --                  "September", "October" , "November", "December"};
  assert(day and month and weekday and year);
  local s = " the ";
  local d = day;
  s = (d=="01" or d=="21" or d=="31") and s..d.."st" or
      (d=="02" or d=="22") and s..d.."nd" or
      (d=="03" or d=="23") and s..d.."rd" or
      s..d.."th";
  print(weekdaynames[weekday]..s.." of "..intmonthnames[month].." in the year "..year);
end

prettyPrintDate(currentday,currentmonth,currentweekday,currentyear);

prettyPrintDate(dateYesterday());