Pressuming your date columns are 52 rows long and the odd intervals are
in ascending order in column A with their data in column B and the 15
minute intervals are in column C, put the following formula in cell
D-1. If column A is not in ascending order use the Sort command from
the Data menu and be sure to sort only column B along with A.

=VLOOKUP(C1,$A$1:$D$52,2)

then copy cell cell D-1 down through D-52


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