I have found at least 13 caches on unique 15th of the month by different hiders
Because the program is greedy with hiders and uses them as soon as you might fulfill the challenge anyways
For example:
2012-01-15 Hider_A GCxxxx1 Cache1
2012-01-15 Hider_B GCxxxx2 Cache2
2012-02-15 Hider_A GCxxxx3 Cache3
Only counts as one found for the program with Hider_A used on 2012-01-15. If Hider_B was used first the result would be two finds
Programing a solution for this problem takes more time than I am willing to spend och the checker
You might be able to find a better solution. You have a total of 57 Hiders on 15 unique days to work on
Only one cache per hider each day is printed below
The checker attempt was:
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2012-12-15 Agent Basil, GC2PDT0 Brush with a View - Two - Two Tango !
2013-02-15 Bethungra Rover Crew, GCPZKA Trooping Bermudas Key
2013-08-15 Eynowd, GCYFK2 Intercepted Transmission
2013-09-15 wing6rider & Crystal Warrior, GC4N69H Dalton-Gunning Earthquake Zone
2013-10-15 eleKtriK turtle, GC4AMVT Re-PlayTime
2013-12-15 CBR900RR, GC1G5BG Round the Board
2014-02-15 Puddle_Jumpers-PJ's, GC2H28Q Bridge over lake Goo
2014-03-15 VikingOlly, GC480YX Masters of the Spin Cycle
2014-04-15 SEMIK, GC3HZXE Five Oh - 45 Pub Lunches
2014-05-15 domsky, GC2C12P Cultural Heritage
2014-08-15 allrounder, GC10XYZ Ainslie's Sheep (BYO pen)
2014-09-15 MajuraHathi, GC3NM92 L'oeuf
2014-10-15 Slipperysally, GC5EKG9 In the Pink
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You might be able to do it better with the following finds:
2012-12-15 Agent Basil, GC2PDT0 Brush with a View - Two - Two Tango !
2012-12-15 mcktcj, GC3XK89 If The Tree Fits....
2012-12-15 RedcrowX, GC388XV Oz Rock - AC/DC: Back in Black
2012-12-15 Tankengine, GC158JD Black Burley
2012-12-15 bubble & squeak, GC2T0BR Peace
2012-12-15 Minifig Ant, GC2VN10 Gaps - Lonsdale St. Braddon
2012-12-15 genghicat, GC3DZXC Loch Be a Lady Tonight
2013-02-15 Bethungra Rover Crew, GCPZKA Trooping Bermudas Key
2013-02-15 Puddle_Jumpers-PJ's and AlanBegado, GC31BW5 Ringwraith North
2013-02-15 Fore!, GC22PE9 Knives and Fore!ks
2013-02-15 marlais and adopted by growingupisoptional, GC115RY To be Encompassed by the Brilliant Earth II
2013-08-15 Eynowd, GCYFK2 Intercepted Transmission
2013-08-15 Fore!, GCYV0R Fore!st Ranger
2013-09-15 wing6rider & Crystal Warrior, GC4N69H Dalton-Gunning Earthquake Zone
2013-09-15 gecko1968, GC4N9R7 Mining the Answers #4
2013-09-15 gecko1968 and Skylight9, GC4J6CC A stones throw away
2013-09-15 CorvettePete, GC2XK0B C1
2013-09-15 Shoobees, GC4J2X3 Canberra Chemistry Cache 8: Oxygen
2013-10-15 eleKtriK turtle, GC4AMVT Re-PlayTime
2013-12-15 CBR900RR, GC1G5BG Round the Board
2013-12-15 Cankid, GC4VATQ We Built This City!
2013-12-15 EdenDanny, GC24C74 Can you sudoku?
2013-12-15 rhino3004, GC239CE Vikings Bowls
2013-12-15 joeylovestreasure and C-chord, GC4EB9Z Stranger on Athllon
2013-12-15 Chalk It Up, GC3HBQF Recycling It Up
2013-12-15 Noikmeister, GC3VNFY A Place for Quiet Reflection
2013-12-15 CaptWheato, GC4V4V4 Stoned
2013-12-15 joeylovestreasure, GC4H6D8 Etruscans
2013-12-15 Matt the Splat, GC2G80Q Big Game Hunt
2013-12-15 Sol de lune, GC23YHY Aussie Wildlife
2013-12-15 rubber.ducky, GC39VPA duck under the bridge
2014-02-15 Puddle_Jumpers-PJ's, GC2H28Q Bridge over lake Goo
2014-02-15 Carojajoda, GC4PP1Z Towrang Bridge
2014-02-15 J+J.au, GC4Q6PF A Sequential Breakfast Puzzle
2014-02-15 O319, GC2N995 Great Southern Road - Bong Bong
2014-02-15 Sc00t (Team Booya), GCMFQN Cecil
2014-02-15 The Bushies, GC42056 "L.K.A Bong Bong"
2014-02-15 NationalSteel, GC4H6GD Cosgrove Park
2014-02-15 Pacmania, GC499B4 Bog's Hollow
2014-02-15 powerplayx2, GC22BYE A Berry Happy Cache
2014-02-15 The Bushies and Sc00t, GC4XFXB "McForest S.B."
2014-02-15 Big Matt and Shell, GC1DYYB Remembrance Driveway - Mackey
2014-03-15 VikingOlly, GC480YX Masters of the Spin Cycle
2014-04-15 SEMIK, GC3HZXE Five Oh - 45 Pub Lunches
2014-04-15 shonkylogic, GC3KX57 A Shonky Powertrail #1
2014-04-15 bructus, GC4ABEX Vigonior cache
2014-04-15 Pop and his Clan, GC2BKMY Alex is Dumped
2014-04-15 Team Ransid, GC1DNCM The Barossa Valley Travel Bug Transit Lounge
2014-04-15 OzFiresafe, GC40WJQ Where is Oz Mega 2014 - Nuriootpa
2014-04-15 The Pilbara Cachers, GC1RB0D Not Through Here
2014-04-15 Grandeeze (Mrs), GC1YKE5 NGADJURI
2014-04-15 Grandeeze, GC2BDAQ The Bear "Rocks"
2014-04-15 PG06, GC18704 SLPO
2014-04-15 Velcro131071, GC3WZJ9 Memorial Of John Fear
2014-05-15 domsky, GC2C12P Cultural Heritage
2014-06-15 Agent Basil, GC1HZJ8 A Brush with a View
2014-07-15 VikingOlly, GC4E74P Silver Tail Road
2014-08-15 allrounder, GC10XYZ Ainslie's Sheep (BYO pen)
2014-09-15 MajuraHathi, GC3NM92 L'oeuf
2014-10-15 Slipperysally, GC5EKG9 In the Pink