To describe an event with a beginning but no end, meaning that it goes forever into the future, one could say it is "endless".

What word would describe an event with no beginning but an end, meaning that it goes forever into the past? 'Startless' doesn't seem to be used for this (or much at all).

In a sentence:

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> They were so used to the custom that they considered its existence [to not have a beginning].
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