This is a Power Query based approach...
I started with this:
Then I added a custom column by clicking the Add Column tab and Custom Column button and completing the pop-up window like this:
...and clicking OK.
Then I changed the type for that new column by selecting it and then clicking the Transform tab and then Data Type and Date.
Then I added another custom column, completing the pop-up like this:
Then I added another custom column, completing the pop-up like this:
Then I added yet another custom column, completing the pop-up like this:
Then I expanded that last column I added by clicking on the at the top of the column and Expand to New Rows.
Then I added a final custom column, completing the pop-up like this:
Finally, I grouped by the Event, DaysLost, Started, and Year columns and summed the DaysLostForYear column by clicking the Transform tab and Group By button and completing the pop-up like this:
I end up with this:
You might want a different grouping, but this should get you close. It shows how many days were lost in the years associated with each instance of an injury's total days lost. For instance, the first injury, which was 30 days in duration, started on 12/25/2016: 7 of those days occurred in 2016 and 23 in 2017. The second injury was 588 days, started on 8/6/2012: 148 days were in 2012, 365 in 2013, and 75 in 2014.
Note that I count the started date as a lost day.
Note also that I account for leap years.
I hope this helps.
Here's the query code:
let
Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Event", type text}, {"DaysLost", Int64.Type}, {"Started", type date}}),
#"Added Custom" = Table.AddColumn(#"Changed Type", "Ended", each Date.AddDays([Started],[DaysLost]-1)),
#"Changed Type1" = Table.TransformColumnTypes(#"Added Custom",{{"Ended", type date}}),
#"Added Custom3" = Table.AddColumn(#"Changed Type1", "DaysYearStarted", each Number.From(Date.From(Text.From(Date.Year([Started]))&"/12/31")-[Started])+1),
#"Added Custom4" = Table.AddColumn(#"Added Custom3", "DaysYearEnded", each Number.From([Ended]-Date.From(Text.From(Date.Year([Ended])-1)&"/12/31"))),
#"Added Custom5" = Table.AddColumn(#"Added Custom4", "Year", each List.Numbers(Date.Year([Started]),Date.Year([Ended])-Date.Year([Started])+1)),
#"Expanded Custom" = Table.ExpandListColumn(#"Added Custom5", "Year"),
#"Added Custom1" = Table.AddColumn(#"Expanded Custom", "DaysLostForYear", each if [Year]=Date.Year([Started]) then [DaysYearStarted] else
if [Year]=Date.Year([Ended]) then [DaysYearEnded] else
if Date.IsLeapYear([Year]) then 366 else 365),
#"Grouped Rows" = Table.Group(#"Added Custom1", {"Event", "DaysLost", "Started", "Year"}, {{"DaysLostForYear", each List.Sum([DaysLostForYear]), type number}})
in
#"Grouped Rows"