Tools -> Language -> English (United States)
Selecting your Language will automatically change the Date format to whichever Date format your country uses.
Then you can Log/Enter Flight Dates as normal.
If this does not work, you will need to highlight the entire Date column. Right click and select the Format which you desire.
For Numbers users, you will likely need to create a custom format.
Highlight the entire Date column. Click Cell. Re-arrange the example dates to suit your format (note: you will need to add either - or / in between the years/months/days.