What's the exact time?

777. Safety Zone for Z’manim, or a Mitzvah Missed in a Moment

The halachic z’manim ([earliest and latest] times [for prayer and other time-bound observances]) are largely based on the position of the sun relative to the earth at different times of day. Sof z’man krias shema (the end-time for the [morning] Shema) is at the end of the third of twelve halachic “hours” of daylight. The actual length of those “hours” in ordinary time depends on both location and season. The Siddur HaRav (compiled by 18th century author of Shluchan Aruch HaRav, Rabbi Schneur Zalman) cautions that a person should recite the morning Shema forty-five minutes earlier than the latest z’man, since ee efshar l’tzamtzem (it is impossible to [measure] precisely); an inaccurate clock may also interfere with calculating the precisely correct time.

In these days of vastly improved clock mechanisms, atomic clocks and other scientific tools for accurately measuring time, one may say that such precautions are unnecessary. However, the many factors that are involved in calculating correct z’manim still limit us—so there are many reasons for creating a “safety zone” by allowing extra time before the z’man:

There are different shitos (opinions) in calculating the correct z’man, resulting in a range of time for both the beginning and the end of the accepted z’man. When it comes to fulfilling a mitzvah optimally, these variations should hold sway.

Each factor of the calculation is rounded off to a small degree for workable numbers, so the resultant time may be inaccurate to some degree, and leeway is necessary to account for it.

Precise location matters, since z’manim are averaged for a geographical area, usually fixed at the city center; outlying areas have slightly different times. Altitude (a mountain or a valley) also plays a part, since one of the factors in calculating the correct z’man is the sun’s angle—precisely to a fraction of a degree—in relation to the horizon.

Atmospheric conditions and weather can also affect the exact measurement of the sun’s position, and these are not taken into account when calculating z’manim based on a regular, yearly solar cycle.

For all these reasons (and others, beyond the scope of a one-minute halachah), z’manim may be inaccurate even amounting to a couple of minutes discrepancy, and it is important to recite the morning Shema before the latest z’man.

A person who waits until the last moment and misses the z’man because of a miscalculation or due to an emergency, is held responsible for procrastinating, as, according to many opinions, ones b’yom ha’acharon lo amrinan (an unwitting [transgressor] on the last day is not considered [unwitting]). Even in the event that a mishap occurred that was beyond his control, and the procrastinator is afforded the label of ones—k’man d’avid lo amrinan ([it is not considered as done); the opportunity was lost for fulfilling a mitzvah.

The same principle applies to all z’manim just as with sof z’man krias shema—both for the latest and earliest times. We should wait a short time before starting Minchah at z’man minchah gedolah (the earliest time at a half-hour after midday) and the earliest time for Maariv at tzes hakochavim (when [three discernible] stars emerge [at night]); it is stated in poskim that we should wait a minimum of few minutes after the z’man before davening Maariv, since we are not baki (expert) in astronomy.

https://halacha2go.com?number=777

Practical Halacha: One minute a day. By Horav Yosef Yeshaya Braun, shlita, Mara D'asra and member of the Badatz of Crown Heights.