I think there is an error in TZ, regarding begin of DST in Portugal in 1980. A user of my website pointed it out. file europe has
Rule Port 1978 only - Oct 1 0:00s 0 - Rule Port 1979 1982 - Sep lastSun 1:00s 0 - Rule Port 1980 only - Mar lastSun 0:00s 1:00 S Rule Port 1981 1982 - Mar lastSun 1:00s 1:00 S Rule Port 1983 only - Mar lastSun 2:00s 1:00 S
But Observatório Astronómico de Lisboa http://oal.ul.pt/hora-legal/legislacao-sobre-a-hora-legal/ has in document Tabela com a definição anual da Hora Legal portuguesa desde 1911 https://oal.ul.pt/documentos/2018/01/hl1911a2018.pdf
1980 Hora do meridiano de Greenwich até à 1 hora do dia 6 de Abril, instante em que foi adiantada de 60 minutos, tendo-se regressado à hiora do meri- diano de Greenwich às 2 horas do dia 28 de Setembro (Portaria nº 69/80)
I attach a patch file