Conditional Formatting Dependant On Date

Edd_Keogh Use this JSON column formatting for your date column: 

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "div",
  "txtContent": "@currentField",
  "style": {
    "background-color": "=if(Number(@currentField) == 0 , '', if(@currentField < addDays(@now, -7), 'red', if(@currentField < @now, 'yellow', 'green')))",
    "padding-left": "14px",
    "color": "=if(Number(@currentField) == 0, '', 'white')"
  }
}

You can also refer the list column values in JSON using the internal name of column like [$StartDate]. You can get the internal name of your column by following this article: How to find the Internal name of columns in SharePoint Online? 

Also, here I have used Number(@currentField) == 0 to check date column is blank or not. You can find other ways to check if date & time column is empty or not at: SharePoint JSON formatting: Check if date & time column is blank/empty

Output: 

You can adjust the conditions as per your requirements if needed.

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