The mishna and gemara require gittin to be dated by the malchus that it is written in. If not, R' Meir holds the gett is pasul and the child born from a second marriage is a mamzer. The Rabbonon argue on R' Meir - According to R' Yehuda in the name of Shmuel they say that if dated by the "santer" [ya'avetz points out that it is the same word as "senator"] then she is divorced b'dieved. According to R' Huna in the name of Rav, even with the date of another malchus, the child would at least be kasher. However, Tosafos asks, that all seem to agree that l'chatchila we must write the date based on that particular malchus, how can we date based on bri'as ha'olam?
Tosafos offers 3 approaches:
1. Rabbeinu Elchonon says that certainly chazal originally required mentioning the king. However, if the king was not mentioned but the santer was mentioned the gett can be used. If it was dated by another malchus the gett cannot be used but if she had a child from another marriage, he is not a mamzer. Based on this, writing from the creation of the world should be passul b'dieved, but things changed since even the non-jews are no longer careful about this. Therefore, even though it is a violation of the original takana, the takana no longer applies.
2. Rabbeinu Yosef suggests that the original takana of the gemara was only to write the date of the malchus in a situation where the malchus is makpid. The original takana is still in existence but includes a dispensation for anything that the malchus is not makpid about. It is not the takana that changes with passing generations, rather the level of kepeida of the malchus changes. Just as writing the santer is kasher because the malchus was not makpid, so too writing bri'as ha'olam nowadays kasher nowadays since the malchus isn't makpid. Maharam Shif points out that even R' Meir would agree since the entire takana even according to R' Meir may have been not to write something that the malchus is makpid on [just that R' Meir and Rabbonon argue whether writing the santer is something they are makpid on].
3. Rabbeinu Tam writes that according to the Rabbonon there was never a takana to write the date based on the malchus at all. R' yehuda in the name of Shmuel is saying that even if one were to write the date based on the santer which could cause animosity from the king, would be kasher l'chatchila. Certainly, writing bri'as ha'olam which will not cause animosity, will be kasher l'chatchila. Based on this, we pasken exactly like the gemara, nothing changed - the gemara was simply saying a chiddush that even if dated by the santer it would be kasher. Our minhag would therefore be based on the Rabbonon according to shmuel, but would not fit with R' Meir.
4. The Ran has another approach that the entire takana was against writing another malchus which would cause jelousy. But there was never a requirement to write the malchus that you are in, therefore if you write based on creation which is benign, it would be kasher even according to R' Meir. Rashi (80a d.h. mi'shum) clearly implies that the institution was to give prestige to the malchus that one resides in, not just a prohibition against writing another malchus, not like the Ran.