What is the earliest time possible that one could wear their tefillin?

ב"ה Shalom, I answered this question some years back , so I will just copy my answer from then with some added notes and references. The Shulchan Aruch (30) speaks about the time that it is permissible to put on Tefillin and describes it as the time "one can recognize a casual friend from a distance of 4 Amot." The are two main opinions today as how to define this time on a clock. According to one opinion it is one hour before "Netz Hachama" = sunrise. Another opinion says 52 minutes before "Netz Hachama".(See פסקי תשובות סי' י"ח:ה, סי' ל':א. ) In difficult situations, if needed to get on your way earlier, you may start Shacharit without your Tallit and tefillin, and plan that when you get to yishtabach ,it should be the correct time to put on Tefillin and then put on your Tefillin. There are other opinions of as well. Some say that these two major opinions are only relevant in Eretz Yisrael , but in other places it must be calculated. There are many websites which deal with Tefilla and Tefillin times around the globe where you can see things in detail. One website which is very useful is called "my zmanim”. All the best