Assuming you have the two lines in two variables, Line1 and Line2 (if you need to, you can read the last line from the file, but easier would be to simply remember the line in a variable each time you write it to the file so you don't have to read it), then the process would be:
- Split the lines in to their component parts.
- Calculate the time in seconds since the epoch.
- Calculate the difference.
- Convert it to hours and minutes.
Each part is relatively straight forward. Use the Search Variable action to split the line apart:
Then you use the TIME function to calculate the number of seconds since 1970 in GMT.
Then you diff them:
The you convert the result to Hours:Minutes as described here:
The result is something like this.
Keyboard Maestro Actions.kmactions (3.0 KB)