import csv
from datetime import datetime
fmt = {"uk_branch": "%d/%m/%Y", "us_branch": "%m/%d/%Y"}
w = csv.writer(open("receipts_iso.csv", "w", newline="")); w.writerow(["receipt", "date"])
for r in csv.DictReader(open("receipts.csv")):
    t = r["date"]
    if r["source"] in fmt: d = datetime.strptime(t, fmt[r["source"]])
    else: d = datetime.strptime(t, "%Y-%m-%d" if "-" in t else "%d %b %Y")
    w.writerow([r["receipt"], d.date().isoformat()])