This page is just a reference of all the known Steam APIs, I do not know how they work. Please do not email me with questions.
PUBLISHERGETGetGameServerPlayerStatsForGame
https://partner.steam-api.com/ISteamGameServerStats/GetGameServerPlayerStatsForGame/v1/
NameValueTypeRequiredDescription
click to setstring Yes access key
uint64 Yes game id to get stats for, if not a mod, it's safe to use appid here
uint32 Yes appID of the game
string Yes range start date/time (Format: YYYY-MM-DD HH:MM:SS, seattle local time
string Yes range end date/time (Format: YYYY-MM-DD HH:MM:SS, seattle local time
uint32 NoMax number of results to return (up to 1000)