This can vary a lot depending on how the date fields are configured in Excel. The first thing you should do is put a compose action in your flow with the date field to see what is being returned. For example, if the dates in your spreadsheet look like 4/21/2023, you want to see if the Compose action returns the value 45037 (45037 equals 4/21/2023). If it does, you are dealing with an Excel serial date.
You can then use a condition to see if the date field is empty. If it is, skip updating the date field in SharePoint. Otherwise you can convert the value to an ISO8601 date format, which is what the SharePoint list requires. You don't show your flow, so you will need to adapt this to your needs:
The expression in the condition to check if the date is null is the following (if your column is not named "Date", you will need to update the expression to your column name:
length(items('Apply_to_each')?['Date'])
The expression to convert the Excel serial date to ISO format is:
addDays('1899-12-30', int(items('Apply_to_each')?['Date']), 'yyyy-MM-dd')
The result:
