This month shall be for you the first of months. It shall be for you he first of the months of the year (Shemos 12:2). The simple meaning of the first phrase is that Nissan is the first month, Iyar the second and Sivan the third (Rashi). If so, the second phrase is merely explanatory (see Da'as Mikra). Seforno explains the first phrase historically: during the long period of slavery, your days were not yours, since you were forced to work for others at their whim. Henceforth your months are yours, to do in them as you wish. As such, the second phrase is a command: because this month is the first in your newfound existence of free will to choose how to spend your time, it shall be for you the first of the months of the year forever.
The transformative transition from bondage to freedom does not merely relieve the excruciating burden of slave labor, rather it ushers in a new reality of freedom to choose how time is spent. To commemorate this transformation and to reinforce its importance, we must begin the count of the months each and every year from Nissan.
Chizkuni inverts the two phrases of the passuk. The first phrase commands that Nissan be counted as the first month (as in Esther 3:7), while the second explains its historical significance: when you count the months starting from the time of your emancipation you will remember the kindness that I did for you. Until now Tishrei was the first month, as it continues to be in the count of years (e.g. for shemita - Devarim 31:10, see Shemos 34:22).
Chizkuni adds that there are two Hebrew words for month - chodesh and yerach. Each month there is a renewal of the light of the moon. Lunar months are only "for you" (lachem), as the other nations count by the sun. The second phrase, which includes "the months of the year," can refer to the need to ensure that Nissan occurs in the spring by adding leap years (see Da'as Mikra).
According to Rav S.R. Hirsch, the renewal of the moon "for you" represents the periodical spiritual and moral rejuvenation of Am Yisrael; Chazal interpret the passuk as "dugma lachem" - the new moon is your model to renew your close connection to Hashem regularly.
The multiple interpretations of the opening of Parshas Hachodesh offer multiple lessons for all of us for all generations. First, we must better appreciate the value of time. As Americans turn attention to a "national pastime," Jews must use their time carefully, and not allow it to pass idly and uselessly. Time is an invaluable and irretrievable commodity, and must be utilized to serve Hashem. When not occupied with basic physical needs and earning a livelihood, a Jew should learn Torah, do mitzvos or practice chessed.
In the modern world, there is more "spare time" than in earlier periods. This time is used in our society for recreation, relaxation and excessive physical indulgence, often leading to levity and immorality. While some relaxation is permissible and sometimes necessary, Jews must dedicate every possible moment to positive spiritual activity. Nowadays, more of our days are ours. A century ago, more hours were needed for a livelihood and household chores. Our newfound existence of more discretionary time is reminiscent of the Seforno's message - your time is yours, use it wisely. We must thank Hashem for placing us in a world of relative affluence and technological progress, in which we have been largely emancipated from the burden of arduous physical labor from dawn to dusk in the field and the home. As the Chizkuni teaches, we count from Nissan to remember the kindness of Hashem.
Finally, as R. Hirsch explains, every new month is a model for us to renew our connection to Hashem. A Jew should neither slide into complacency, nor become despondent in the contemplation of spiritual mediocrity or worse. Rejuvenation in the form of greater devotion to serving Hashem is the lesson of Parshas Hachodesh. As we personally renew and strengthen our commitment to Torah, mitzvos and chessed, we pray for national renewal as well. In Nissan we were redeemed and in Nissan we will merit the ultimate redemption.