What have been the dates on which the state of alarm in Spain for Covid-19 has been extended?

The state of alarm was first declared on March 14th under Ley Orgánica 4/1981 - on states of alarm, exception and siege. Article Six of Chapter II states:

One. The declaration of the state of alarm will be carried out by decree agreed in the Council of Ministers.

Two. The decree will determine the territorial scope, the duration and the effects of the state of alarm, which may not exceed fifteen days. It may only be extended with the express authorization of the Congress of Deputies, which in this case may establish the scope and conditions in force during the extension.

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Because the initial declaration & the extensions all required Royal Decrees, this makes each extension easier to pinpoint. The initial declaration was enacted by Royal Decree 463/2020, of March 14. At the end of this page, there are references to the six other Royal Decrees which extended the state of alarm - these are Royal Decree 476/2020, of March 27, Royal Decree 487/2020, of April 10, Royal Decree 492/2020, of April 24, Royal Decree 514/2020 of May 8, Royal Decree 537/2020, of May 22, and finally, Royal Decree 555/2020, of June 5. The state of alarm ended at 00:00 on June 21st.