You can retrieve the first and last QSO dates for a registered callsign using this API:
https://clublog.org/logcheck.php
Parameters are passed with HTTP GET and are as follows:
call = The callsign of the log
api = Your Club Log API key
The returned string is a JSON array of two elements. The first element is the first QSO date/time and the second is the last QSO date/time. The values may be null, which means the log exists but is empty. If you receive a 404, the callsign is not registered in Club Log.
For example:
https://clublog.org/logcheck.php?call=gd7vjr&api=[your API key]
...returns:
["2009-04-03 06:56:00","2009-07-25 11:44:00"]
https://clublog.org/logcheck.php?call=FT9ABC&api=[your API key]
...returns HTTP status code, 404, not found.