The Natal Chart is about ascertaining the future from the time of birth. But suppose one is planning an important event or project and especially wants it to be successful. Logically, he should choose the best time available within a particular "window of time," to get the best results. This is called Muhurta or Electional astrology. Basically it is the science of timing, choosing the best time to start something in order to get the best result. There are certain events in life that we would especially like to see turn out successful, such as a business or marriage. If one has undergone all the trouble to match charts for compatibility, then you should also choose a good muhurta for the wedding, in order to start at the right time and encourage a favorable outcome.
With a little imagination, we can expand the possibilities for muhurta astrology well beyond these two items, business and marriage: When to build a house, make a significant purchase, start any important project like writing a book, when to release a book or movie, or when to begin a long journey. Vedic Kings would consult their astrologers regarding when to begin a military campaign or sign a peace treaty. Vedic parents would consult as to when to conceive a child and when to perform the child's samskaras (sacraments). Gurus and acaryas (great spiritual leaders) would choose auspicious muhurtas for conducting an initiation ceremony, installing a Deity, or laying the cornerstone of a temple (Example: Solar and lunar eclipses are considered very good for initiations.) Vaidyas (Ayurvedic physicians) would choose auspicious times for performing surgery or beginning a regimen of treatment (Example: Since the moon rules over fluids, it is considered very undesirable to have surgery near the full-moon day, because of the tendency for excessive loss of fluid and danger of hemorrhaging.)
Here is an example of a practical application of muhurta astrology. In 1992 some clients of mine had to make an important presentation to the board of directors of an important spiritual organization. By studying the charts of the men involved and the time slot available, I chose what I considered to be a perfect time. They had previously wanted a much later time, but I put pressure on them to accept the date that I had chosen. Somehow they were able to arrange their presentation for the appointed day, and it went off perfectly. They later told me that in the days following their presentation, the mood of the directors had changed drastically, so had they waited until the time originally planned for making the very same presentation, it was very doubtful that they would have been able to carry it through.