@staticmethod<EOL><INDENT>def timestamp_update(sender, frames):<DEDENT>
for frame in frames:<EOL><INDENT>frame.modified = datetime.now(timezone.utc)<EOL><DEDENT>
Timestamp the modified field for all documents. This method should be
bound to a frame class like so:

```
MyFrameClass.listen('update', MyFrameClass.timestamp_update)
```