Sifrei Devarim Paragraph 256
Can copy this into an AI, or connect the whole library to your assistant. For deeper learning, you can directly use the original seforim, sites, and apps.
(דברים כג יב) וְהָיָה לִפְנוֹת עֶרֶב יִרְחַץ בַּמָּיִם, מְלַמֵּד שֶׁהַקֶּרִי פּוֹטֵר בַּזִּיבָה מֵעֵת לְעֵת.
(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.)
וּבָא הַשֶּׁמֶשׁ, בִּיאַת שִׁמְשׁוֹ מְעַכַּבְתּוֹ לִיכָּנֵס לִפְנִים מִן הַמַּחֲנֶה וְאֵין זִיבָתוֹ מְעַכַּבְתּוֹ לִיכָּנֵס לִפְנִים מִן הַמַּחֲנֶה.
"and when the sun sets": The (non-) setting of the sun prevents him from entering the camp.