date and date/time matching ad averaging

This workbook (attached) has date/hour timestamp in column A, so usually you'll see 24 rows per calendar day. Column B contains the values attached to column A.

I want to match the calendar days (column C) to the values in column A, and then in column D, we want to average the values in column A.

The formula is not working correctly.

Could someone either fix the formula, or show me how to write one which accomplishes the same outcome?

I've manually averaged the first two days, which display the correct values in Column D.

I will appreciate your help!

Note: I have hidden rows 1 to 440 because they will not be associated with this task.

Thanks so much William and John. I went straight to John's formula, which works. It also occurred to me that if I want some other metric such as trimean or median, I could rearrange the formula so it begins with an If statement. (=if date in C is... , trimean). If I did that would it reduce CPU time since its not an array?

Or try:

=IFERROR(AVERAGEIFS($B$441:$B$1000,$A$441:$A$1000,">="&C441,$A$441:$A$1000,"<"&C441+1),"")

Normal enter.

Thanks everyone for your help.