Hi. I've tried searching for an answer to this but have come up empty handed. Perhaps someone can help.

I want to calculate time difference between several values AND then be able to SUM those times (I want daily and monthly totals for time on level 1, time on level 2, and total). Unfortunately, I will have blanks in my sheet.

When I use a conditional statement such as IF(ISBLANK(CELL) to calculate 1 total value, I cannot then use that cell in another formula for monthly sum without getting a #value or #### error.

I am creating this spreadsheet so I am open to suggestions on how best to format it.

Example of my data:
1. Date/Time Level 1 On: 12/2/2013 18:54
Date/Time Escalated to Level 2: blank/empty cell
Date/Time Off: 12/2/2013 22:54

2. Date/Time Level 1 On: 12/3/2013 12:50
Date/Time Escalated to Level 2: 12/3/2013 13:36
Date/Time Off: 12/3/2013 21:31

3. Date/Time Level 1 On: 12/4/2013 16:27
Date/Time Escalated to Level 2: 12/5/2013 02:29
Date/Time Off: blank/empty cell