Actually, for this I would use date vectors. You can then use ismember with the 'rows' option to locate the matching rows of minute and daily data (using only the year month day columns of the date vectors):
DailyDatenum = [734504:734868; 1:365]';
MinuteDatenum = [734502:1/24/60:734870]';
Dailydv = datevec(DailyDatenum(:, 1));
Minutedv = datevec(MinuteDatenum(:, 1));
[ispresent, row] = ismember(Minutedv(:, 1:3), Dailydv(:, 1:3), 'rows');
MinuteDatenum(ispresent, 2) = DailyDatenum(row(ispresent), 2);
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