Cost basis of inherited mutual fund

I have read that the cost basis of an inherited mutual fund is stepped up to the value of the fund on the date of the decedent’s passing. Is the correct date to use in ascertaining the cost basis 1) the date of death, or 2) the date(s) the shares are actually transferred into the heirs’ names? In the case at hand, my spouse and her three siblings inherited a mutual fund which had belonged to their mother. The brokerage handling the transfer took 5 or 6 months to complete the process, and not on the same date for all heirs (each heir was handled separately). Plus the fund re-invested a dividend each month. So it would be much simpler if the “date-transferred” date is used instead of the date of death.