The answer to this question can be found in the first section of this novel, just after George and Lennie leave the natural grove they discover at the beginning of the story that is a sight of such natural beauty and tranquility. After describing the bunkhouse without anybody in it, suddenly the owner enters with George and Lennie. Note the description that the text gives us just before they enter:
At about ten o'clock in the morning the sun threw a bright dust-laden bar through one of the side windows, and in and out of the beam flies shot like rushing stars.
We can therefore see that after the scene set at night time, or thereabouts, in the grove, Lennie and George actually arrive at the bunkhouse at around ten o'clock in the morning, obviously having managed to gain some work.