Date time diff in Excel - excel-formula

Question

I would like assistance in calculating the difference of date and time between each of cells 1 through 15 in hours and minutes.
The problem is the ref is not the same for all so, when the ref changes from R001 to R002, the formula should start calculating the date time difference from the first cell, with the new reference to the last cell with the same ref.

Answer 1

This solution will use AGGREGATE, INDEX and optionally MINUTE functions built into excel
Step 1
Build a list of unique ref IDs and place them in column D
Step 2
Use AGGREGATE to determine the LAST row that a ref ID occurs in.
AGGREGATE(16,6,ROW($A$2:$A$16)/--($C$2:$C$16=D2),1)
AGGREGATE is a function that performs array like calculations. As such avoid using full column references within the function.
16 tells AGGREGATE to use the LARGE formula, so it will built a list in Descending order.
6 tells AGGREGATE to ignore/exclude any results in the list that are an error.
ROW() is going to pull the row number that is being evalutated.
-- will turn the resulting TRUE or FALSE result into 1 or 0.
C2:C16=D2 is looking to to see if the row has the REF ID .
1 tells AGGREGATE to return the 1st number in the list.
Step 3
Rinse, wash, repeat. Determine the FIRST row that a ref ID occurs in.
Again use aggregate just as before. Except this time instead of LARGE, we want to find SMALL so change the 16 to a 15. This will return the row number of the first occurrence of the REF ID.
AGGREGATE(15,6,ROW($A$2:$A$16)/--($C$2:$C$16=D2),1)
Step 4
Now that you know the ROW# to use, place the row number in an INDEX formula to get the cell you want to use. Note that index actually returns the cell address so can be used just like a cell reference.
First Time Refence
INDEX(B:B,AGGREGATE(16,6,ROW($A$2:$A$16)/--($C$2:$C$16=D2),1))
Last Time Reference
INDEX(B:B,AGGREGATE(15,6,ROW($A$2:$A$16)/--($C$2:$C$16=D2),1))
Step 5
Take the difference between the Last and First
INDEX(B:B,AGGREGATE(15,6,ROW($A$2:$A$16)/--($C$2:$C$16=D2),1))-INDEX(B:B,AGGREGATE(16,6,ROW($A$2:$A$16)/--($C$2:$C$16=D2),1))
Now if the first reference occurs before the time stamp of the last referenec you will wind up with a negative number. IF YOU DO NOT CARE ABOUT THE NEGATIVE NUMBER, wrap the whole thing in an absolute function or square the results then square root the results.
ABS(INDEX(B:B,AGGREGATE(15,6,ROW($A$2:$A$16)/--($C$2:$C$16=D2),1))-INDEX(B:B,AGGREGATE(16,6,ROW($A$2:$A$16)/--($C$2:$C$16=D2),1)))
Place your choice of the above formula in E2 and copy down as required.
Step 6
Since you want your answer in JUST hours and minutes, I opted to break the information into two separate columns so you could use them as needed. Also I will be referencing the values in column E instead of embedding them into the following formulas which is also an option if you want to avoid extra columns.
If you format the results in E2 as general you will note you get an integer and some decimal values. The integer represents the number of days. The decimals represent TIME as fraction of a day. In order to get the number of hours, you multiply by 24, since there are 24 hours in a day, and take just the integer. Place the following formula in F2 and copy down.
INT(24*E2)
Since you also want the minutes we can pull those directly from E2 using the MINUTE function. Place the following in G2 and copy down.
MINUTE(E2)