But that's not historically what the terms mean---though some popular defenders of the Reformed view occasionally make this mistake, whether through ignorance or oversimplification.
Historically, the Reformed understanding is that Christ's "passive obedience" and his "active obedience" both refer to the whole of Christ's work. The distinction highlights different aspects, not periods, of Christ's work in paying the penalty for sin ("passive obedience") and fulfilling the precepts of the law ("active obedience").
Louis Berkhof puts it in his standard Systematic Theology:
The two accompany each other at every point in the Saviour's life. There is a constant interpretation of the two. . . .
Christ's active and passive obedience should be regarded as complementary parts of an organic whole. (pp. 379, 380)
John Murray, in Redemption---Accomplished and Applied, expresses it quite clearly and goes into more detail:
[We cannot] allocate certain phases or acts of our Lord's life on earth to the active obedience and certain other phases and acts to the passive obedience. The distinction between the active and passive obedience is not a distinction of periods. It is our Lord's whole work of obedience in every phase and period that is described as active and passive, and we must avoid the mistake of thinking that the active obedience applies to the obedience of his life and the passive obedience to the obedience of his final sufferings and death.
The real use and purpose of the formula is to emphasize the two distinct aspects of our Lord's vicarious obedience. The truth expressed rests upon the recognition that the law of God has both penal sanctions and positive demands. It demands not only the full discharge of its precepts but also the infliction of penalty for all infractions and shortcomings. It is this twofold demand of the law of God which is taken into account when we speak of the active and passive obedience of Christ. Christ as the vicar of his people came under the curse and condemnation due to sin and he also fulfilled the law of God in all its positive requirements. In other words, he took care of the guilt of sin and perfectly fulfilled the demands of righteousness. He perfectly met both the penal and the preceptive requirements of God's law. The passive obedience refers to the former and the active obedience to the latter. (pp. 20-22)
Put another way, Jesus' so-called "passive" and "active" obedience were lifelong endeavors as he fulfilled the demands and suffered the penalties of God's law, and both culminated in the cross. I would argue that the New Testament clearly teaches the lifelong passive obedience of Christ (his penalty-bearing work) and the lifelong active obedience of Christ (his will-of-God-obeying work), culminating in the cross. We then receive the benefit of this through the imputation of the obedience of Christ (the reckoning of Christ's complete work to our account when we trust in him for salvation and are united to him).
But it's not uncommon to hear some people say, "Scripture only teaches the imputation of Christ's passive obedience, not his active obedience." Another complaint goes like this: "Such fine distinctions are owing more to systematic theology than to the authorial intent we're after in exegetical theology."
But here's the irony in it all. By arguing that it is only Christ's passive obedience that is imputed to our account, they are the ones making a distinction that can't be found in the biblical text.
The Reformed folks argue for both-and, not either-or. In the NT Christ's righteous work is of one cloth: it's always "obedience unto death." In other words, one cannot separate Christ's fulfillment of God's precepts from Christ's payment of the penalty for failing to obey God's precepts.
What God has joined together, let no man separate!