It is possible to celebrate two days of Purim as follows: Celebrate Purim on the 14th of Adar, as it typical, and then travel to a "walled city" where Purim is celebrated on the 15th of Adar, and celebrate Purim there again. My question is: what does our tradition say about celebrating zero days of Purim (by reversing the above travel).
The Mishna Brurah (688:12) says:
בן עיר שהלך לכרך אם היה דעתו בעת נסיעתו לחזור למקומו בזמן קריאה ר"ל בזמן קריתה של ט"ו דהיינו משהאיר היום לא יהיה שם כי יחזור משם קודם אור היום אף שלבסוף רואה שהוא מוכרח להתעכב שם גם על יום ט"ו מ"מ אינו נקרא בשם מוקף וקורא ביום י"ד בהיותו בכרך
A non-walled city dweller who travels to a walled city: if he intended when he traveled to return to his place at the time of reading - that means the time of reading on daybreak of the 15th of Adar he doesn't intend to be there because he will return, even though in the end he sees that he is unable to leave and has to stay over on the 15th, nevertheless he is not called a "walled city person" and he reads on the 14th while he is in the walled city.
So the upshot is that, according to the Mishna Brurah, if one tries to do that, they have to read the Megillah (i.e. celebrate Purim) on the 14th inside the walled city.
The Yalkut Yosef (Kitzur S"A 688:8) says that one who lives in Yerushalayim and leaves on the 14th after dawn and stays away (in Tel Aviv) until after 15th at dawn, has successfully avoided Purim - however, he says that anyone G-d fearing should not do this, and if he did, he still has to read Megillah and do the other Mitzvos on the 15th, he just does it without a Bracha and cannot read Megillah for others.
In :9 he goes on to say that most poskim hold that once you celebrated Purim on the 14th, you have no obligation to do so on the 15th, unless you actually moved to Yerushalyim the night of the 15th. But Rav Shlomo Zalman Aurebauch disagrees, so it is praiseworth to hear Megillah on the 15th if you go to Yerushalaim the night of the 15th and stay until morning.
According to Rabbi Eliezer Melamed (end of footnote 7), citing the Rosh, one's location on the 14th of Adar at Alot HaShachar is the only determinate of which day you are obligated to celebrate Purim. As such one can never be absolved of the obligations of celebrating Purim through travel.