It's not just birthdays, anniversaries or significant days of the calendar.
Consider also the ubiquitous counters on social media sites; the use of biometric technology in personal exercise regimes; the goal-oriented logic of structured weight-loss programs; the steady stream of statistics and sporting records published each week; the significance that accrues around the first 100 days of the US presidency.
In these and a multitude of other ways, we impose a kind of order on the passage of time.
Time is, after all, the way that we measure change, and change is the way we give some sense to the passage of time.
As Aristotle put it, "not only do we measure change by time, but time by change, because they are defined by one another."
Landmarks in time
Milestones, therefore, could be thought of as temporal landmarks.
Much like the way physical landmarks demarcate meaningful locations, temporal landmarks demarcate meaningful junctures in time.
They interpose themselves between the past and a hoped-for future — or, again to crib Aristotle's terms, between our actuality and our potentiality.
Research in psychology bears this out, demonstrating that we perceive temporal landmarks, like decade birthdays or New Year's Days, as divisions between a past self and the present self who has the future in reach.
This division helps guide goal-pursuit behaviours and gives meaning to our efforts.
An inescapable social dimension
Does it matter that all such milestones are arbitrary measures? Not really, because the very the fact that most milestones are socially agreed upon means they are not arbitrary.
It's not the number associated with a milestone that matters per se, but the psychological and social processes that are expressed by recognising these agreed-upon achievements.
In other words, milestones have an inescapably social or relational dimension to them.
The measurement itself frequently reflects a common value, and the achievement of the goal acts as an invitation to shared action.
That is why we often come together around milestone achievements, like graduations or significant birthdays, which provide an occasion for social bonding around a positive event.
It is this, perhaps, which distinguishes birthdays and graduations from collective commemorations that take place around annual holidays, whether religious or secular.
There is something unique about milestones in that they are achievements.
An opportunity to attain prestige
It's probably not too much of a stretch to make the connection between milestones and the pre-modern notion of honour, which was an objective status that could not be seized but could only be conferred by the adulation of others.
Milestone achievements are socially-agreed-upon contexts in which we confer and attain prestige.
In the psychological literature, prestige (status earned through the possession of skills and qualities respected by society) is contrasted with dominance (status earned through aggression and intimidation).
Interestingly, milestone achievements also offer a context in which we feel the positive emotion associated with success: pride.
Arresting the flow of time
This is not the whole picture.
What about milestones that are not necessarily moments of pride, like 50 years since the 1967 referendum, which saw appallingly discriminatory phrases removed from the Australian constitution, or 75 years since the massacre at Babi Yar, or 100 years since the start of the First World War?
Such milestones don't mark achievements, but they do arrest the flow of time, even if for a moment, and allow us to give time elapsed a kind of moral inflection: how have we — as individuals, as a society, as a community of nations — changed in the interim?
Has the passage of time transformed us, or have we squandered the time given us?
And this points to a conception of time that seems to have become increasingly foreign to us.
Time in a life well-lived
The predominant way we think about time these days is as a scarce commodity that we dare not squander and therefore must pack with achievements, experience or profit.
We could call this a capitalist conception of time.
But then there is the teleological or purposive notion that time is the gift given to us during which to pursue a "complete life" — a profoundly Aristotelian notion bound up with the cultivation of virtue and the achievement of happiness. We would probably call it a life well-lived.
From this second perspective, milestones aren't simply goals that we achieve.
Rather, they're points to which we and our friends are summoned to return, from which we learn and deepen the quality of our life together, and through which to rededicate ourselves to the pursuit of goals and goods in common that will outlive us.
This is getting close to Hannah Arendt's description of the longing for immortality that is present in any genuinely public act — the willingness to use our time in order to attain a good that will exceed the time given us.
Our own milestone
Recently our own little radio show reached a milestone of sorts, passing its 100th episode.
Many shows have been going for much longer. All the same, there is every reason to be proud — and a little amazed — that the show has lasted as long as it has, precisely because it is so intellectually demanding to produce and expects so much from its audience.
The Minefield is a strange creature indeed in a media-saturated age characterised by pre-packaged analyses, uncivil disagreements, instant thinkers and received ideas.
But if there is one thing 100 episodes has taught us, it's that beneath even the most mundane topics lie vast intellectual treasures and great moral dangers. The show is, after all, called The Minefield.