(Devarim 21:12) "And it shall be towards evening that he shall wash himself with water": This teaches us that a seminal emission exempts a zivah (seminal) discharge within the same twenty-four hour period (from the requirement of cleansing, the verse implying that whatever intervenes, he will be clean in the evening.) "and when the sun sets": The (non-) setting of the sun prevents him from entering the camp.
Sifrei Devarim 256
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