Can someone help me with writing a unix script for following requirement

1) I have a log file in which we have start time and end time (format: hh:mm:ss)
Example: starting script on Thu Jun 5 20:50:52
Thu Jun 5 21:55:33 - Script Completed
2) I want to extract start time and end time of the script (ex: 20:50:52 and 21:55:33) from the log file and write into a excel file.
3) I need to collect these start and end times every day and write into the excel (this excel will be stored in a shared drive) and also I have around 50 logs which i need to go thru to get the start and end times.
4) Once the start and end times are extracted into excel file, I need to get the time taken for completion of the script (by substracting end time from start time) and populate the result value into another cell of the excel.
For example, My ouput should look like this.
Start Time End Time Time to Complete
19:41:42 20:25:18 00:43:36

Your help with this script is greatly appriciated.

Thanks,
S