I don't understand the ruling at the first, because a doesn't mean one in front of many, few . For your question, either a or one is correct.
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coloraday Anyhow, the question still stands; how can 'a' be used instead of 'one' in such a sentence?Because in your sentence "a" is equivalent to "one".
coloradaythe closest OALD gets is 'used instead of one before some numbers'.Maybe we'll have to write in to the editors of the OALD and ask them to expand their definition.
coloradayI'm looking for the reason why it's equivalent to 'one' in these time expressions. I can't seem to find such usage of 'a' instead of 'one' in any dictionary; the closest OALD gets is 'used instead of one before some numbers'.Look at it this way: