Depending on your location and spreadsheet's settings Google Sheets will expect a certain format for your dates. In the image below our spreadsheet is set to United States so Google Sheets expects a Date format of MM/DD/YYYY.
Cell G2 & G4 are recognized as dates, but cell G3 & G5 aren't.
G3 is seen as just text because it is not in the date format the spreadsheet is expecting
G5 is seen as a number as we originally entered 05292009 with no spaces.
You can change the settings for your 'Locale' and affect the way your dates are formatted in Sheets. Go to File/Spreadsheet Settings and you'll see a window like the one below.
It's possible to change your 'Locale' and not affect your timezone. In the image below our timezone is London GMT but the Locale is United States.