COMMENTARY on 17:12
A. Yusuf Ali:

Translation:
We have made the Night and the Day two 2185 (of Our) Signs: the Sign of the Night have We obscured, while the Sign of the Day We have made to enlighten you; that you may seek bounty from your Lord, 2186 and that you may know the number and count of the years: all things have We explained in detail.
Commentary:

2185  If we were to cry when it is night, we shall look foolish when it is day; for the night is but a preparation or the day: perhaps, as the last verse says, we pray for the day when we want rest for the night. Both are Signs from Allah. Darkness and light stand for ignorance and knowledge. "Where ignorance is bliss, 'tis folly to be wise." Darkness and light may also stand for shadow and sunshine, sorrow and joy: both may be necessary for our development.

2186  By the physical light we see physical facts. And this physical gift of Allah is good for us in two ways; (1) we can arrange for our livelihood, or we can attain knowledge of the physical sciences and gain some control over the physical forces of nature; and (2) the daily rising and setting of the sun gives us the computation of days and years, for the physical natural year is the solar year. But there is spiritual light even more precious: by it we can similarly attain two objects, viz. (1) our spiritual livelihood and knowledge, and (2) our computation of the stages we reach in our spiritual years. Let us be patient and seek everything as from Allah-in joy and in sorrow, in knowledge and in want of knowledge of those things which are above us. Let us rejoice in what Allah has given us, and not be impatient about those things which He in His wisdom has thought fit to withhold from us. But all things should be sought and striven for under the guidance of the All-Knowing Allah.

 

Muhammad Asad:

Translation:
And We have established the night and the day as two symbols; 13 and thereupon We have effaced the symbol of night and set up [in its place] the light giving symbol of day, 14 so that you might seek to obtain your Sustainer's bounty and be aware of the passing years 15 and of the reckoning [that is bound to come]. For clearly, most clearly, have We spelt out everything! 16
Commentary:
13  Regarding the primary meaning of the term ayah, see note 2 above. In the present context, the expression ayatayn ("two symbols") refers - as the subsequent clause shows - to the symbols of spiritual darkness and light.
14  I.e., the message of the Qur'an, which is meant to lead man out of spiritual ignorance and error into the light of faith and reason.
15  Lit., "the count (`adad) of years". Since, as the Qamus points out, this phrase denotes also "the years of [a person's] life, which he counts", it obviously implies here a call to spiritual self-criticism in view of the ephemeral nature of one's worldly life.
16  Le., everything that man may be in need of in the domain of ethics and religion.