How could I transform the YYYYDDD date string/Integer to YYYYMMDD date string?

Question

suppose I have lots of YYYYDDD date Integers which look like(for example):

dateslalala = 
Outer[1000 #1 + #2 &, {2003, 2005, 2004}, 
RandomInteger[{0, 365}, 20]] // Flatten

gives:

{2003067, 2003280, 2003362, 2003021, 2003223, 2003115, 2003049, 
2003233, 2003341, 2003074, 2003134, 2003182, 2003332, 2003061, 
2003184,...}

I want to transform them to the style YYYYMMDD like "20010101" or integer 20010101, Err... I didn't figure out have to do this by function like DateString quickly. In fact I use this ugly cumbersome code:

 StringJoin /@ ({ToString@#1, StringTake["00" <> ToString@#2, -2], 
 StringTake[
  "00" <> ToString@#3, -2]} & @@@ ((y = Floor[#/1000.]; 
   d = Mod[#, 1000]; DateList[{y, 1, d}][[1 ;; 3]]) & /@ 
 dateslalala))

gives:

{"20030308", "20031007", "20031228", "20030121", "20030811", 
"20030425", "20030218", "20030821", "20031207", "20030315", "20030514"...}

is there any more natural way to do this?

Answer 1

Perhaps, using:

dateslalala={2003364, 2003157, 2003314, 2003302, 2003181, 2003062, 2003254, \
2003070, 2003365, 2003338, 2003233, 2003073, 2003020, 2003010, \
2003238, 2003107, 2003310, 2003347, 2003204, 2003066, 2005364, \
2005157, 2005314, 2005302, 2005181, 2005062, 2005254, 2005070, \
2005365, 2005338, 2005233, 2005073, 2005020, 2005010, 2005238, \
2005107, 2005310, 2005347, 2005204, 2005066, 2004364, 2004157, \
2004314, 2004302, 2004181, 2004062, 2004254, 2004070, 2004365, \
2004338, 2004233, 2004073, 2004020, 2004010, 2004238, 2004107, \
2004310, 2004347, 2004204, 2004066};

then,

qr = QuotientRemainder[#, 1000] & /@ dateslalala;
DatePlus[{#1 - 1, 12, 31}, #2] & @@@ qr

yielding:

{{2003, 12, 30}, {2003, 6, 6}, {2003, 11, 10}, {2003, 10, 29}, {2003, 
  6, 30}, {2003, 3, 3}, {2003, 9, 11}, {2003, 3, 11}, {2003, 12, 
  31}, {2003, 12, 4}, {2003, 8, 21}, {2003, 3, 14}, {2003, 1, 
  20}, {2003, 1, 10}, {2003, 8, 26}, {2003, 4, 17}, {2003, 11, 
  6}, {2003, 12, 13}, {2003, 7, 23}, {2003, 3, 7}, {2005, 12, 
  30}, {2005, 6, 6}, {2005, 11, 10}, {2005, 10, 29}, {2005, 6, 
  30}, {2005, 3, 3}, {2005, 9, 11}, {2005, 3, 11}, {2005, 12, 
  31}, {2005, 12, 4}, {2005, 8, 21}, {2005, 3, 14}, {2005, 1, 
  20}, {2005, 1, 10}, {2005, 8, 26}, {2005, 4, 17}, {2005, 11, 
  6}, {2005, 12, 13}, {2005, 7, 23}, {2005, 3, 7}, {2004, 12, 
  29}, {2004, 6, 5}, {2004, 11, 9}, {2004, 10, 28}, {2004, 6, 
  29}, {2004, 3, 2}, {2004, 9, 10}, {2004, 3, 10}, {2004, 12, 
  30}, {2004, 12, 3}, {2004, 8, 20}, {2004, 3, 13}, {2004, 1, 
  20}, {2004, 1, 10}, {2004, 8, 25}, {2004, 4, 16}, {2004, 11, 
  5}, {2004, 12, 12}, {2004, 7, 22}, {2004, 3, 6}}

Answer 2

The thought is DatePlus.

split[x_Integer] := {{FromDigits@#[[1 ;; 4]], 1, 1}, 

    FromDigits@#[[5 ;; -1]]} &@IntegerDigits[x]

split[2003305]

(*
    {{2003,1,1},305}
*)

DatePlus[split[2003305][[1]], 305]

(*
    {2003,11,2}
*)

f = Block[{$DateStringFormat = {"Year", "Month", "Day"}, res}, 

    res = split[#]; DatePlus[res[[1]], res[[2]] - 1]] &;

f /@ dateslalala

(*
    {{2003,11,1},{2003,7,1},{2003,2,4},{2003,5,25},{2003,10,9},{2003,12,21},{2003,10,17},{2003
,10,30},{2003,9,18},{2003,1,14},{2003,3,12},{2003,10,14},{2003,11,4},{2003,1,14},{2003,12,
13},{2003,1,10},{2003,8,11},{2003,9,26},{2003,2,14},{2003,1,12},{2005,11,1},{2005,7,1},{20
05,2,4},{2005,5,25},{2005,10,9},{2005,12,21},{2005,10,17},{2005,10,30},{2005,9,18},{2005,1
,14},{2005,3,12},{2005,10,14},{2005,11,4},{2005,1,14},{2005,12,13},{2005,1,10},{2005,8,11}
,{2005,9,26},{2005,2,14},{2005,1,12},{2004,10,31},{2004,6,30},{2004,2,4},{2004,5,24},{2004
,10,8},{2004,12,20},{2004,10,16},{2004,10,29},{2004,9,17},{2004,1,14},{2004,3,11},{2004,10
,13},{2004,11,3},{2004,1,14},{2004,12,12},{2004,1,10},{2004,8,10},{2004,9,25},{2004,2,14},
{2004,1,12}}
*)

Answer 3

I am not sure I understand the constraints of this question if there are any. But would this work:

  Map[ Function[
  DateString[DatePlus[
    DateList[{ToExpression[StringTake[ToString[#1], 4]], 1, 
      1}], {(ToExpression[StringTake[ToString[#1], -3]] - 1), 
     "Day"}], {"Year", "Month", "Day"}]], dateslalala]