Logbook of the World
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Developer Information: Querying LoTW for a List of Users with Last Upload Information 

LoTW provides a web service at https://lotw.arrl.org/lotw-user-activity.csv that returns a file in comma-separated format, each of whose lines specify a callsign, date of last upload, and time of last upload, e.g.

1A0C,2016-04-23,20:15:52
1A0KM,2014-02-08,15:08:08
1A3A,2008-09-12,22:44:57

Dates are UTC, specified in YYYY-MM-DD format.

Times are UTC. specified in HH:MM:DD format.

Lines are separated by a single line feed character (0x0A).

The contents of this file are typically updated once each week.