A PRIMER ON “HALACHIC TIME”
We have learned that the appropriate time for the Hefsek Tahara is after the time called Plag HaMinchah. (Lit. “the middle of the mincha”). There are a number of available web sites that will provide sunrise and sunset information for any place on earth; there are a few that will provide you with the actual time of plag haminchah. My goal here is to remove some of the confusion that resulted from an oral explanation of the concept.
The Halachic day consists of 12 hours of daylight and 12 hours of night, all year round. Therefore, to compute halachic times, you need to adjust the length of each hour – basically, total daylight minutes divided by 12 gives you the length of one halachic hour.
For example:
In Minneapolis on August 17, 2004 sunrise is at 6:18 AM and sunset is at 8:15 PM or 20:15. We can compute the amount of daylight as follows:
Sunrise 
6:18 
Sunset 
20:15 
Daylight Hours 
13:57 
So on this date, there will be 13 hours and 57 minutes of daylight, which is 837 minutes.
Therefore, the length of a halachic hour is computed as follows:
Minutes 
837 
Divide into 
12 
Min/Hr 
69.75 
On this date, then, a halachic hour is 69 ¾ minutes.
Now we can begin to make calculations that will give us all sorts of times that are useful in halacha. For example, one can say Shema until 3 hours of the day, one can pray until 4 hours and one can begin to say the minchah prayers at 6 ½ hours. There is a second time for mincha that is 9 ½ hours of the day. This is what is called minchah ketana, or the “small minchah”. This is generally considered the better point at which to do minchah during the day. Finally, the time we are most interested in – plag haminchah – is half way in between minchah ketana and sunset. Since that period is 2 ½ hours, the midpoint is 1 ¼ hours after minchah ketana, or 10 ¾ hours of the day.
We can now calculate all of these times.

Shema 
Prayer 
Earliest Minchah 
Minchah Ketana 
Plag HaMinchah 
Minutes/Hr 
69.75 
69.75 
69.75 
69.75 
69.75 
Hours 
3.00 
4.00 
6.50 
9.50 
10.75 
Minutes 
209.25 
279.00 
453.38 
662.63 
749.81 
Std. Hours 
3.49 
4.65 
7.56 
11.04 
12.50 
In Time format 
3:29 
4:39 
7:33 
11:02 
12:29 
Sunrise 
6:18 
6:18 
6:18 
6:18 
6:18 
Time 
9:47 
10:57 
13:51 
17:20 
18:47 
All we’ve done is multiply the number of minutes per halachic hour by the appropriate number of halachic hours relevant for each time. The result is the total number of minutes after sunrise that the event occurs. We convert that back into hours and minutes and add that to sunrise time. Then we are left with the appropriate time for all of the various events listed. There are ways of shortcutting this calculation in Excel, but I have avoided them.
Therefore, on August 17, 2004, a woman can perform her hefsek taharah from 6:47 PM until sunset at 8:15 PM.
Note: All of these calculations are according to the views of the Vilna Gaon and represent the primary view of time calculation. There are other views, primarily those of the Magen Avraham that are not reflected here.