What is the ruling on fulfilling an oath made regarding the beginning of the lunar month?

Primary text

If one swears to fulfill an obligation at the beginning of the crescent moon, with its beginning, until the beginning of the crescent moon, until its appearance, at the beginning of the month, or with its beginning, and fulfills it at sunset on the eve of that month, the oath is fulfilled (he is clear of it). If he delays this fulfillment when capable, he violates his oath (Hanth). This ruling is supported by the position of Al-Shafi'i regarding these matters.