Not too familiar with the API's yet, but one option if there is not one that just gives the answer would be to simply use the game archive endpointhttps://www.chess.com/news/view/published-data-api#pubapi-endpoint-games-archive for the last 3 months for that player and then iterate over each game looking at the rating for the player in question.
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Is there any way that we can note the peak/highest rating (blitz/rapid) of a player in a given period of time (Like from the last 30/60/90 days)? If so let me know (For spreadsheets).