To determine your biorhyhthms for today follow these steps:

1. Enter your date of birth;
2. Click the button which says [Calculate Biorhythms].

Wait a few seconds and you will get the graphic of your biorhythms for today. If you wish to have it for a later date, first you should change the "Date for the biorhythms" before step 2.

By clicking the button [Reseet dates for today], you get today's biorhythms for those who were born today. When one is born, one is physically, emotionally and intellectually in the critical point (zero in the graphic above).

According to the theory of biorhythms, everything repeats itself in cycles in nature and man is no exception. According to the same theory, our physical cycle lasts for 23 days, the emotional one lasts for 28 days and the intellectual one lasts for 33 days, being the counting made from your date of birth.

The graphic above shows a line in the middle that runs completely from left to right. The values above that line (from 0% to 100%) are positive ones and below it (from 0% to 100%) are negative ones. Those values which are on the line (0%) are called critical or semi-critical points. Those days, which are neither positive nor negative, are days of instability when in one moment we are sad and in the next moment we are happy (emotional). In those days we shouldn't close a deal, for example, according to this theory.

The highlighted vertical bar marks today and on its right the following days. It is this bar that sets the percentual values which we read below and on the left for our physical, emotional and intellectual condition. Still below and on the right of the bar we have the scale with the following days of the month. In the scale above, today's day is marked with a zero. This scale is adequate for an analysis of the kind "in x days" and in particular in one, two or three weeks, signaled by the pink vertical bars.

If you haven't entered any date yet (if you have already done it, click the button [Set dates for today]), the graphic will show the biorhythms for those who were born today. When one is born all the three curves are in one point. One is physically, emotionally and intellectually in one semi-critical point (semi because it is an ascending one). Seven days after it (pink vertical line) one is emotionally 100% (green curve) and almost 100% on the others.

Below is a small extract of the complete text which you can find here:
http://en.wikipedia.org/wiki/Biorhythm

The classical theory originated at the turn of the 19th century, between 1897 and 1902, from observational research.

Hermann Swoboda, professor of psychology at the University of Vienna, who was researching periodic variations in fevers, looked into the possibility of a rhythmic change in mood and health. He collected data on reaction to pain, outbreak of fevers, illnesses, asthma, heart attacks, and recurrent dreams. He concluded that there was a 23-day physical cycle and a 28-day emotional cycle.

Wilhelm Fliess, a nose and throat specialist and reportedly a numerologist, was independently researching the occurrences of fevers, recurrent illnesses and deaths in his patients. He too came to the conclusion that there was a 23 and a 28-day rhythm. Fliess' theories were of great interest and importance to Sigmund Freud during his early work in developing his psychoanalytic concepts.

Alfred Teltscher, professor of engineering at the University of Innsbruck, observed that his students' good days and bad days followed a rhythmic pattern of 33 days. Teltscher found that the brain's ability to absorb, mental ability, and alertness ran in 33 day cycles. In the 1920s, Dr. Rexford Hersey (psychologist; Pennsylvania, America) also reportedly made contributions to the classical theory.

These three biorhythms compose the classical theory. The classical theory has been studied, especially in Germany, Japan, and the United States, with conflicting results. Various modern derivatives exist of the classical theory.