For the source text click/tap here: Pesachim 107
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The Rambam appears to have understood our beraisa to be explaining when the earliest time is to daven mincha but provides two times. One, mincha gedolah, is the earliest possible time, whereas the other is the preferred time. In other words, the earliest time to daven mincha is at 6½ hours, although it is preferred for someone to wait until 9½ hours to daven mincha. This is because it is ideal to daven mincha later in the day and closer to sunset. This affects the laws of erev pesach