I was told in chat that this is where I can report an MS Office 365 Excel bug.
When opening a CSV file that includes any field with data in mm:ss format, Excel correctly interprets data from 00:01 to 23:59 as mm minutes and ss seconds. However any data from 24:00 to 59:59 is misinterpreted as mm hours and ss minutes (i.e. x60).
It is illogical, and therefore must be a bug, to interpret 23:59 correctly as 23 minutes 59 seconds, and to interpret one second more in the data; i.e., 24:00, as 24 hours zero minutes.
Try saving the following actual data from Garmin Connect into a file named "Activities.csv" and opening it with Excel:
Activity Type,Start,Time,Distance,Avg Pace,Avg HR,Max HR,Calories,
Street Running,"Thu, Jun 26, 2014 8:22 AM",56:08,4.00,14:03,126,159,478,
Casual Walking,"Sun, Jun 22, 2014 9:21 AM",18:15,0.96,18:59,91,117,98,
Casual Walking,"Thu, Jun 19, 2014 6:32 PM",51:41,1.99,26:02,94,113,209,
Street Running,"Thu, Jun 19, 2014 8:45 AM",55:52,4.00,13:59,137,171,489,
I have a workaround: When using the data, since time data is stored as days (and anything greater than 23 minutes and 59 seconds is stored as 1 or more days), test the field; i.e., IF(value>=1,value/60,value). For example to convert from time data to the number of minutes, use the following formula "=1440*if(value>=1,value/60,value)".
Bottom line: I don't need any answer except how to get Microsoft to recognize this illogical behavior as a bug that will be fixed, eliminating the need for a workaround.