Ancient Jewish tradition holds that days begin at sunset - a practice codified in religious law and observed for millennia. Yet a careful examination of biblical and historical sources reveals this wasn't always the case. The earliest biblical chronometry actually counted days from sunrise to sunrise, with nights belonging to the preceding day - a finding that challenges fundamental assumptions about Jewish time-keeping. Through comprehensive analysis of biblical passages and rabbinical sources, particularly those concerning Passover observance and Yom Kippur, clear patterns emerge that contradict the traditional interpretation of Genesis 1's "evening and morning" formula. Close reading of scriptural references, Temple service protocols, and Mishnaic texts provides compelling evidence: Passover eve was designated as the fourteenth day of Nisan despite occurring at night, while Temple services operated exclusively during daylight hours. These findings demonstrate that the now-universal practice of counting days from evening to evening emerged as a post-Temple period adaptation rather than reflecting original biblical precedent. The transition appears driven by changing social and economic conditions following the Temple's destruction, exemplifying how halakhic practice evolved through historical adaptation rather than unchanging divine ordinance. This reconstruction of Jewish chronometry's development offers insights into halakhic evolution as a dynamic process of continuous revelation shaped by historical circumstances.