How to calculate days between two dates from different sheets where the emails from both sheets in row match?

I have 2 sheets in same workbook that have dates that I need to calculate the number of days between the two dates. There is a common identifier in both sheets rows being email. If there is 0 days between dates it should state 0 and a date is missing it should showing blank ""

https://docs.google.com/spreadsheets/d/1tigqy4hKFn0Q7c-3ICyI6WREnsIBFZ2oLO8CEcdxe8w/edit?usp=sharing

results: Sheet1!J:2 start date Sheet1!D:D End date: Day_count!B:B Matching identifier = email in Col1 on both sheets

What would be the best way to work this out without using a helper column

Answer This works when the lookup isn't in Col1

={"Day count"; ARRAYFORMULA(IFNA(DAYS(VLOOKUP(A2:A, {Days_count!B2:B, Days_count!C2:C}, 2, 0), D2:D)))}