Google Flights

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gfl
Examples: 
gfl txl, lpa, fr, 30
Search for round trips from Berlin Tegel to Las Palmas, departing next Friday, returning on the 30th of this month
URL: 
https://www.google.com/flights/#flt={%from IATA code|transform=uppercase}.{%to IATA code|transform=uppercase}.{%departure date|type=date}*{%to IATA code|transform=uppercase}.{%from IATA code|transform=uppercase}.{%arrival date|type=date}