Tevul Yom, Chapter 4, Mishnah 4. A tevul yom is a person or a vessel that has already been immersed in a mikveh but whose purification is not yet complete: only when the sun sets and the day ends does the taharah take full effect. Until then the tevul yom still carries a measure of tumah, enough to disqualify terumah that comes into contact with it. Our Mishnah applies this to a case where a man wants to separate terumas ma'aser, and the only pitcher he has on hand is itself a tevul yom.
Here is the situation the Mishnah lays out. "Lagin shehu tevul yom": a jug that has already been immersed in a mikveh and is now waiting only for the day to end before its taharah is complete. "Umil'ohu min hechavis ma'aser tevel": into this jug he draws wine from a large storage container of ma'aser produce whose terumas ma'aser, the share due to the kohen, has not yet been removed. The contents of that container are therefore still undivided: nothing yet marks off which of the wine will remain plain ma'aser and which will become the kohen's portion.
At this point he makes a verbal designation: "im amar, harei zo terumas ma'aser mishetechshach, harei zo terumas ma'aser." Should he declare, "What is in this jug shall become terumas ma'aser once darkness falls," the designation is valid and that wine really is terumas ma'aser. Why should the hour he specifies make such a difference? A designation meant to take effect immediately, while it is still day, would accomplish nothing: terumas ma'aser held in a vessel that is still waiting for sunset would be rendered unfit by the very vessel containing it, since that vessel's purification is unfinished. Once he phrases the designation to begin only after dark, however, the problem dissolves. By that hour the jug has finished its purification and is entirely tahor, so the terumas ma'aser comes into existence inside a vessel that cannot pasul it.
The Mishnah then presents a second declaration that does not work: "im amar, harei zeh eiruv, lo amar klum." If he says, "Let this be my eiruv," meaning an eiruv techumin, food set down before Shabbos so that a person may walk beyond his two thousand amos on Shabbos, he has accomplished nothing. Terumas ma'aser can in principle serve as an eiruv, but only if it is tahor, and this wine will not be tahor until nightfall. Meanwhile, an eiruv techumin must be established while it is still Friday, while it is still day. So his single declaration is pulling in two directions at once: he needs the wine to be terumas ma'aser that is tahor, which happens only after dark, and he needs the eiruv to be in place before dark. The two cannot be reconciled, and therefore the designation fails entirely.
The Mishnah closes with two consequences of this arrangement: "nishberah hechavis, halagin betivlo; nishbar halagin, hechavis betivlah." If the barrel breaks, the wine drawn off into the pitcher remains tevel, since the separation was never completed. And if the pitcher breaks, the wine left in the barrel likewise remains tevel. The designation was meant to fix the status of both portions together, and if either one is lost, the other stays in its untithed state.
A small pitcher, a barrel of wine, and a setting sun: the Mishnah teaches us how precisely a person's words can be aimed. Say it for now and the tumah of the vessel ruins it; say it for nightfall and it works perfectly, unless the halachah you are trying to satisfy insists on daylight.