Background

The problem I'm solving is mentioned here in detail. Another glitch is that the spreadsheet I'm reading from has random changes to the date format like so:

10/3/2021   11/3/2021   12/3/2021   13-03-2021  14-03-2021

When I import these dates into another sheet, how can I do it so that the dates are normalized (ie are imported in the same format)?

B/c what's happening is that when I'm extracting these dates and I run a chart on them, the chart only reads the first format and ignores the rest:

Spreadsheet details

I get the spreadsheet by email as an .xlxs file. I open it and save it as a google spreadsheet, which has locale as the United States.

I made a copy of the spreadsheet here.

What I tried

I simply tried applying a date format of DD/MM/YYYY on the cells with the different date format but that's not working. What's weird is that in the sheet I'm importing from, it's giving me a different date format than what appears:

Right now all I'm doing is manually changing dates from the DD-MM-YYYY format to DD/MM/YYYY Format.

I also tried converting the dates using this formula

=to_date(int(A1))

but it worked with some cells not others