To those who stubbornly claim that there is "no basis for this 'chumra' " and try to convince other not to be concerned over this inyan which impact shmiras shabbos -
Below are TWO concurring sources of a posek hador and a renown Talmid Chochom, Rav S.Z. Auerbach and Harav Moshe Shternbuch. I accept these as better halachic authorities than Health, b'michilas kvodo.
The seforim of these gedolim are cited below and can be looked up by anyone who can read Hebrew without nekudos.
Is it proper to extend greetings of Shabbat Shalom (Gut Shabbos) on Friday prior to Mincha?
HaGoan HaRav Shlomo Zalman Auerbach (ZL)was not in favor of extending such a greeting prior to Mincha. His rationale was that it is well known that Hagoan HaRav Akiva Eiger ruled that a person who said on Friday night “Shabbat Shalom” may, by such a statement, observe the Biblical mitzvah of Kiddush which is a requirement to simply remember the Shabbat on Shabbat, [Zachor et Yom HaShabbat]. To observe the Mitzva of Kiddush implys that one has accepted upon himself the observances of Shabbat. Accordingly, such a person would not be permitted to Daven a weekday Mincha; for one cannot observe the Kiddush of Shabbat and then contradict such a status by davening a weekday Mincha. Such contradictory observances are not permissible.Though it is not assumed that the Halacha follows HaRav Akiva Eiger’s above noted rule, at the same time, one may not simply assume that his ruling is rejected.
Indeed, the stature of HaRav Akiva Eiger is so great that it is proper to be concerned with his halachic positions. It was, therefore, recommended to greet friends prior to Mincha by saying,”a Gut Erev Shabbos”. It was intimated, moreover, to recite the word “Erev”in a subdued voice. (V’Alaihu Lo Yibol, Volume I, Orech Chayyim 191, pp. 136-137)
Harav Moshe Shternbuch, without citing a source, notes the reputed view of Harav Auerbach that it was improper to say either “ Gut Shabbos or Shabbat Shalom” prior to Mincha on Friday due to HaRav Akiva Eiger’s ruling that such a greeting may be a form of observing the Biblical Mitzva of ‘’Zachor et Yom HaShabbat”. HaRav Auerbach suggests that one need not be concerned with this issue during the daytime on Friday for several reasons. First of all the greeting of ‘Gut Shabbos” does not imply that the person is at that moment accepting Shabbat. It merely means that one wishes another to have or experience a joyous Shabbat. Also, one certainly does not have intention to observe the Biblical Mitzva of Kiddush by the greeting of “Gut Shabbos”.[The Halacha is that Biblical Mitzvot require intention (Kavana) for observance. See Shulchan Aruch Orech Chayyim 60:4] He suggests that HaRav Akiva Eiger’s ruling is operational only on Friday night once Shabbat has commenced. At that period of time, the theory is that since Shabbat has already commenced and the person says “Gut Shabbos”, perhaps, that greeting is a form of observing the Kiddush of Shabbat. However, prior to the onset of Shabbat there is no indication that the greeting in any way is an acceptance of the onset of Shabbat. As a pragmatic custom HaRav Shternbuch suggests that ab initio [l’chat’chila] one should refrain from saying “Gut Shabbos or Shabbat Shalom’’ prior to Mincha for , perhaps, one may have intention that Shabbat has commenced.(Teshuvot V’Hanhagot, Volume IV, Siman 59)
The Mishna Berura rules that prior to “Plag HaMincha”-[an hour and fifteen minutes prior to sunset on a day wherein there are twelve daytime hours], the Mitzva of Kiddush is not operational. (Mishna Berura, Orech Chayyim, 263:18 and 261:25) Accordingly, prior to that time it should be permissible to greet others by saying “Shabbat Shalom or Gut Shabbos for since Kiddush does not take place there should be no apprehension of any observance other than a greeting that Shabbat should be a happy experience.
Rabbi J. Simcha Cohen
Now you know WHEN it's time to say Gut Shabbos...as Reb Abie Rotenberg sings.