At the end of the story they isolate us from them and prevent us from reaching the stars, I don't think the implication is that this is just due to the weird calendar system. I think its more than that, our inability to rid ourselves from our tradition and religious heritage has more profound implications. We don't look at things rationally and make decisions to improve ourselves. The aliens may have gone through similar steps in their evolution of their civilization so they probably know what it *means* if a post-interstellar species still uses a gods birthday to count time.
We aren't making any collective rational decisions yet, we build elaborate structures around tradition that more or less managed to reduce our collective human suffering at least in the northern hemisphere. It took us hundreds of years to rid ourselves from slavery and it will probably take us hundreds more to figure out that murdering billions of animals is bad for example. We might never figure out how to prevent poverty, inequality, racism, imperialism, colonialism and war. We probably make earth uninhabitable way before then anyhow.
I can see why aliens want to isolate and not want to talk to us. Not because of the calendar system we use, but because of what it represents.