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You are correct and it is at a point in time. I think in your original follow up you were trying to determine the value each day as the maximum value. I was trying to point out the value at a point in time which is the greatest in the year would be sufficient and the account statements as back up for audit purposes would suffice to make that determination.
Determine the maximum value of each account (in the currency
of that account) during the calendar year being reported. The maximum
value of an account is a reasonable approximation of the greatest value
of currency or non monetary assets in the account during the calendar
year. Periodic account statements may be relied on to determine the
maximum value of the account, provided that the statements fairly
reflect the maximum account value during the calendar year. For Item
15, if the filer had a financial interest in more than one account, each
account must be valued separately
So you should go through the account statements and provide the maximum balance for the 12 month period. You don't have to do a valuation for each day or an average over the year.