While hardheartedness (v. 31) is a standard metaphor, the contexts we have just seen make it particularly apt. Given the agricultural metaphor of seeds, his listeners could easily see the difficulty of sprouting on hard ground. Amulek tells them to soften their hearts and plant the seed.
The reference to the “time and the day” I read as continuing the agricultural allusion to season and (presumably) propitious days of the month. Amulek, unlike a farmer’s almanac, however, is urging them to take immediate action rather than waiting for some other time. If they “immediately” (in this season and day) plant the seed of faith, then his atonement “immediately” applies to them.
But is this literally true? Must they not do other things in addition to declaring Christ’s name? Of course, but that is not the issue. Amulek is describing a relationship between a king and his people. The king possesses the right to dispense the benefit of the atonement and will do so for his people—but not until they have taken the first step by taking upon themselves his name. Benjamin proposed the process for his own people (Mosiah 5:1–9).
Verse 32 simply reiterates, though in memorable terms, the theme of urgency. There is no other time when we might repent. Today is the time. The phrase “the day of this life is the day for men to perform their labors” is likely another allusion to the propitious days for planting familiar to his farmer-listeners. Amulek has told them that it is “day” and that they must therefore “perform their labors” (plant the seed of faith) before night falls (death comes). The repetition in verse 33 would simply underscore to his receptive audience that they must plant the seed of faith “in the day” so that “night” will not find them excluded from the king’s community. Amulek uses the day/life—night/death metaphor to stress the need to take prompt action, a need he continues by implication in the next verse.