Is a vow considered an intention? And is one who intended to fulfill a vow on a specific day, but did not explicitly designate it, then postponed fulfilling it to another day, sinful?

The promise relates to the intention of the one making the promise, based on the Prophet's (PBUH) saying: "Deeds are only by intentions." Fulfilling a promise is desirable, not obligatory, according to the majority of scholars; hence, there is no sin upon whoever does not fulfill it.