Timestamp when only one option on dropdown menu occurs, with multiple occurrences on one row?

Hey, all!
I found a formula wherein a timestamp will occur when a chosen option is selected on a drop down. For example, when "Blocked" is selected, no timestamp shows up, but when "Complete" is selected, one does occur. Seemed to work fine until I introduced another occurrence on the same row, following the same rules, for the end of the procedure (Ultimately, I'm trying to time the duration between both drop downs displaying "Complete".) Once the second occurrence was added, both time stamps started updating when a dropdown selection was made, even though the one formula was pulling from the "start" drop down and the other was pulling from the "end". It seemed like the NOW() formula just started a timer that kept running, once "Complete" was selected. Any ideas on how to achieve this idea? Thanks in advance!