I make two manual entries of average times to compare last year with this
year. I need to calculate the difference between the two times (it could be
an increase or decrease), and show both the difference (+ or -) in hours and
minutes, and the % of change (+ or -) from last year to this year.

Example: 2:03 average talk time last year, 3:50 average talk time this year
= increase of 1:47 (1 min. 47 sec.). or 86.99 % increase in talk time.