SB 11.21.9

karmaṇyo guṇavān kālo
 dravyataḥ svata eva vā
yato nivartate karma
 sa doṣo ’karmakaḥ smṛtaḥ
Synonyms: 
karmaṇyaḥ — suitable for executing one’s prescribed duty; guṇavān — pure; kālaḥ — time; dravyataḥ — by achievement of auspicious objects; svataḥ — by its own nature; eva — indeed; — or; yataḥ — due to which (time); nivartate — is impeded; karma — one’s duty; saḥ — this (time); doṣaḥ — impure; akarmakaḥ — inappropriate for working properly; smṛtaḥ — is considered.
Translation: 
A specific time is considered pure when it is appropriate, either by its own nature or through achievement of suitable paraphernalia, for the performance of one’s prescribed duty. That time which impedes the performance of one’s duty is considered impure.
Purport: 

Having discussed pure and impure places, the Lord now discusses different qualities of time. Certain times, such as the brāhma-muhūrta, the last few hours before sunrise, are always auspicious for spiritual advancement. Other times, not auspicious in themselves, become so by achievement of material prosperity that facilitates one’s mission in life.

Political, social or economic disturbances that obstruct the execution of one’s religious duties are considered inauspicious times. Similarly, a woman is considered contaminated just after childbirth or during her menstrual period. She cannot perform ordinary religious activities at such times, which are therefore inauspicious and impure. Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura explains that the most auspicious of all times is the moment one achieves the mercy of the Supreme Personality of Godhead. If one neglects the loving service of the Lord, being carried away by sense gratification, he is certainly living in most inauspicious times. Therefore that moment in which one achieves the association of the Supreme Lord or the Lord’s pure devotee is the most auspicious time, whereas the moment of losing such association is most inauspicious. In other words, the perfection of life is simply Kṛṣṇa consciousness, by which one transcends the dualities of time and space caused by the three modes of material nature.