Seeking a bit more information and offering a caution...
Questions:
1.) Do you have a unique identifier for each employee in both lists? If so, are those unique identifiers the same name and same value in both lists? This will play into the 'Get items' later.
2.) Do you already have a calculated column in the Events list to give you the total days (DateDif) between the [Start date] and [End date] of that discrete event? This will give you an integer with the 'TotalDays' for that event.
Caution:
I would caution you against using an automated (based on an event like an item being created) flow versus using a scheduled flow. If a line item in your Events list gets deleted, your Totals list will not be updated to reflect the correct number of days for each employee.
Suggestion:
1.) Create a Scheduled Flow based on a timeline that makes sense for your environment (once per hour, once per day, etc.)
2.) Assuming you have the same unique identifier for each employee across both lists, start with a 'Get items' action that will return the Totals SharePoint list. We'll name it 'Get Totals List Items'
3.) Initialize an integer variable
4.) Add an 'Apply to each'. We'll name it 'Apply to each employee'. In the 'Select an output...' field, insert dynamic content of 'value' from the 'Get Totals List Items'.
5.) Insert another 'Get items' action to return the Events list with a filter query that returns only records with a matching employee ID (dynamic content from the previous 'Get items'). We'll name it 'Get Events List Items'
6.) Create a 'Set variable' where you reset the counter variable (from step 3) to 0. Move this action above the second 'Get items' but still within the 'Apply to each employee' loop
7.) Add another 'Apply to each'. We'll name it 'Apply total from each event'. In the 'Select an output...' field, insert dynamic content of 'value' from the 'Get Events List Items'.
8.) Insert 'Increment variable' using the same initialized variable from above and insert the dynamic content of the (assumed to exist) 'TotalDays' field from the 'Apply total from each event' step. The hover-over text will look something like this: items('Apply_total_from_each_event')?['TotalDays']
9.) Just outside the 'Apply total from each event' loop, add a SharePoint 'Update item' action and select the Totals SharePoint list. In the 'Id', select the matching 'Id' field from 'Get Totals List Items' step. In your 'Total Days' field, insert the dynamic content of your variable.
All this assumes you will have the same employees in one list as you do the other. No error checking has been built into this flow. It may be something to consider should there be an employee in one list but not the other.
I've checked the flow with some dummy data in my sandbox, so I'm happy to help troubleshoot if this goes awry.
- Jerry
