Those dates are derived from the zero crossing method which incorporates an extreme in solar torque measurements.
Then Carl Smith in 2007 using JPL data and his own programming skills plotted the Angular Momentum of the Sun.
The oncoming Grand Minimum will prove Carl's graph is the key to solar activity, which will radically change the solar scientific arena.
Jose's paper included a very rough solar radius graph which showed some modulation but was difficult to draw from.
Later Theodor Landscheidt wrote many papers using a similar principle but mainly relied on solar torque graphs which ranged over long time periods.
There are others that suspect the Sun is modulated by the planets with a special emphasis on Uranus & Neptune.
Thanks to Carl Smith who has recently left us we have new knowledge that significantly adds to Jose, Landscheidt & Carl is no longer with us, but he has certainly left us with a legacy.