Question: What engine was Dota 2 based on at the start of September 2015?
Paraphrase: Give the name of the engine  the Dota 2 was based on as of September 2015.
SPARQL (Wikidata): SELECT ?obj WHERE { wd:Q771541 p:P408 ?s . ?s ps:P408 ?obj . ?s pq:P582 ?x filter(contains(YEAR(?x),'2015')) }