def last_modified(self, name: str = None) -> str:
if name is not None:<EOL><INDENT>return self[name].last_modified()<EOL><DEDENT>ts = "<STR_LIT>"<EOL>for name in self.keys():<EOL><INDENT>if ts is None:<EOL><INDENT>ts = self[name].last_modified()<EOL><DEDENT>else:<EOL><INDENT>if self[name].last_modified() > ts:<EOL><INDENT>ts = self[name].last_modified()<EOL><DEDENT><DEDENT><DEDENT>return ts<EOL>
Return a compact ISO8601 timestamp (UTC timezone) indicating when the layer was last modified

Note: if name is None, the modification time of the most recently modified layer is returned