Well no, the problem is I very frequently have to interface with APIs that give me human readable time strings only. Those APIs were not written by top quality

Well no, the problem is I very frequently have to interface with APIs that give me human readable time strings only. Those APIs were not written by top quality engineers and I often have no other choice or way to interface with those systems. Also you mention ISO-8601 but this document does not even describe how to input an ISO-8601 string.