"Happy birthday, Synthia!"

"Why," mused Synthia, "do we celebrate birthdays? I fail to see anything special about the calendar day on which a person's age-in-calendar-years passes an integer. Are things supposed to be different now that it's been seven-point-five-seven-four times ten-to-the-eighth seconds since my birth, rather than seven-point-five-seven-three?"

Quiana suppressed a groan. Somehow she had been expecting Synthia to say something "normal," like Thanks! But knowing Synthia, it would have been unusual if she had done so: relative to Quiana's state of knowledge of her friend, this—the general style, though not, of course, the specific questions—was the normal response.

Quiana said, "Is that a rhetorical question or a genuine one? Surely you can think of a few reasons yourself."

"I'd like to hear what you think."

"It's true that time itself is continuous and linear, and that there's nothing intrinsically special about a particular interval of time—whatever intrinsically would mean in this context. But we live on a planet with cyclic variation in weather and daylight, so it's natural to think in terms of days and years, and therefore, 'this day last year.' And even if things were otherwise—if our culture had matured on some sort of space station with no such natural cycles—integer units are indispensably useful; we need them in order to make sense of continuous quantities. We'd just end up celebrating our analogues of anniversaries every million minutes or whatever, for the same reason you just named intervals of time to merely four significant figures and in seconds, rather than using an arbitrary number of figures in some arbitrary units."

"That's not what I meant," said Synthia. "Of course I understand the importance of units, and I agree that if we're going to celebrate particular people for no reason, then it's not surprising that we do it on birth anniversaries—I can't think of a better Schelling point. Rather, my question is this: if a world without celebrations is too terrible to imagine, why do we have celebrations for no reason other than the calendar date?—this applies to regular holidays as much as birthdays. Why not reserve celebrations strictly for when someone actually accomplishes something?"

"Like ... a school graduation party."

It was just the first example that came to her mind, but Quiana was not surprised to see a brief flicker of pain cross Synthia's face.

"Yes, things like that," Synthia said.

"Well, if that's what you think, how do you justify the Christmas—"

"Newtonmas."

"—Newtonmas party you had here three days ago?"

Synthia shrugged and made a tight-lipped expression that Quiana could only interpret as meaning Yes, I'm a worthless hypocrite like everyone else; tell me something I don't know.

"But anyway," said Quiana, "even if you think people should earn their parties somehow, no one's throwing you an explicit celebration today; I didn't even get you a gift. So when I said 'Happy birthday, Synthia!' two minutes ago, why didn't you just say 'Thanks,' as if I had said 'Happy Wednesday!' or 'I hope you're having an excellent interval between eleven-thirteen and noon-minus-pi-minutes'?"

"Oh, is that what you meant?"

"... yes?"

Synthia beamed. "Thanks."