I'm inclined to say you should just scrap this function entirely and do

```
# Register cron and ISO8601 date-time format checkers with the global checker.
g_format_checker.checks('cron', raises=(ValueError,))(croniter.croniter)
g_format_checker.checks('date-time', raises=(iso8601.iso8601.ParseError))(from_timestamp)
```

It's less code and more explicit about the expectations of the functions being used as checkers.  Alternatively write some very explicit is_cron and is_timestamp checker functions.