If one believes that our world is billions of years old, i.e. accepting the idea of each day in Sheshis Imei Berieshit (the Six Days of Creation) as eras and not literal days, then how does one understand keeping Shabbat as the seventh day, thus taking that 'day' literally?
Also, one can consider that "time started" in the way we know it only at the end of creation with the creation of Shabbat. Thus, only Shabbat has to be considered a 24 hour day. For example the sun moon and stars were created on the fourth "day" so that "before" then could not have been days as we know them. Before Yom HaShishi, each day was turned on and off explicitly. It is only at the very end (Vayechulu) do we have the actual time being significant.
Also consider that only man has an appreciation of time. This could imply that "time started" only with the creation of Adam. This could lead to a hashkafa article, but that is too long and philosophical to go into now.
Rabbi Dovid Gottlieb suggests that in the original six days of creation time moved faster than today. just like in the embryonic stage one does not need to breathe through his nostrils or eat through his mouth, etc.
so you can have a 24 hour day spanning billions of years the rules were different in those embryonic days
(we also find for example Kain and Abel born and grown up in the sixth day according to the midrash)
ויאמר משה אכלוהו היום, כי-שבת היום ליהוה: היום, לא תמצאוהו בשדה. ששת ימים, תלקטוהו; וביום השביעי שבת, לא יהיה-בו
And Moses said "eat [the Manna] today, as today is a Sabbath for G-d. Today, you will not find it in the field. 6 days collect it, and on the seventh rest, for it will not be there.
This verse cannot be interpreted to refer to an era. It is clear from this verse that the Sabbath is one day. So when we are told to rest on the seventh day, it is clear that it is one day.
A clearer verse (which happens to be in Shabbos Day kiddush according to many customs) - Shemos 20:7-9 (the aseres hadibros):
זכור את-יום השבת, לקדשו. ששת ימים תעבוד, ועשית כל-מלאכתך. ויום, השביעי--שבת, ליהוה אלוהיך:
Remember the Sabbath to sanctify it. 6 days work, and do all your activities, and on the seventh day, rest for Hashem your G-d.
Again, here clearly this is not referring to eras, and this is the injunction to keep the Sabbath. It also refers to the Sabbath as the seventh day.