POSIX time values.

Ptime has platform independent support for POSIX time. It provides a type to represent a well-defined range of POSIX timestamps with picosecond precision, conversion with date-time values, conversion with RFC 3339 timestamps and pretty printing to a human-readable, locale-independent representation.

Ptime_clock provides access to a system POSIX clock and the system's current time zone offset. Ptime is not a calendar library.

Consult the basics and a few notes and limitations.

**References**

- The Open Group. The Open Group Base Specifications Issue 7, section 4.15 Seconds Since the Epoch. 2013
- G. Klyne et al.
*Date and Time on the Internet: Timestamps*. RFC 3339, 2002.

*v0.8.3 - homepage*

`type span`

The type for signed picosecond precision POSIX time spans. A value of this type represent the POSIX duration between two POSIX timestamps.

`val v : (int * int64) -> t`

`v s`

is `of_span (Span.v s)`

but

- Raises Invalid_argument: if
`s`

is not in the right range. Use Span.of_d_ps and of_span to deal with untrusted input.

`val max : t`

`max`

is 9999-12-31 23:59:59.999999999999 UTC, the latest timestamp
representable by Ptime.

`val of_float_s : float -> t option`

`of_float_s d`

is like of_span but with `d`

as a floating point
second POSIX span `d`

. This function is compatible with the result
of Unix.gettimeofday. Decimal fractional seconds beyond `1e-12`

are truncated.

`val to_float_s : t -> float`

`to_float_s t`

is like to_span but returns a floating point second
POSIX span.

`truncate ~frac_s t`

is `t`

truncated to the `frac_s`

decimal
fractional second. Effectively this reduces precision without
rounding, the timestamp remains in the second it is in. `frac_s`

is clipped to the range [`0`

;`12`

].

**WARNING.** A POSIX time span is not equal to an SI second based
time span, see the basics. Do not use these functions
to perform calendar arithmetic or measure wall-clock durations,
you will fail.

`type tz_offset_s`

` = int`

The type for time zone offsets between local and UTC timelines in seconds. This is the signed difference in seconds between the local timeline and the UTC timeline:

```
tz_offset_s = local - UTC
```

- A value of
`-3600`

means that the local timeline is sixty minutes*behind*the UTC timeline. - A value of
`3600`

means that the local timeline is sixty minutes*ahead*the UTC timeline.

A *date-time* represents a point on the UTC timeline by pairing
a date in the proleptic Gregorian calendar and a second precision
daytime in a local timeline with stated relationship to the UTC
timeline.

`type date`

` = int * int * int`

The type for big-endian proleptic Gregorian dates. A triple
`(y, m, d)`

with:

`y`

the year from`0`

to`9999`

.`0`

denotes -1 BCE (this follows the ISO 8601 convention).`m`

is the month from`1`

to`12`

`d`

is the day from`1`

to`28`

,`29`

,`30`

or`31`

depending on`m`

and`y`

A date is said to be *valid* iff the values `(y, m, d)`

are
in the range mentioned above and represent an existing date in the
proleptic Gregorian calendar.

The type for daytimes on a local timeline. Pairs a triple ```
(hh,
mm, ss)
```

denoting the time on the local timeline and a `tz_offset`

stating the relationship of the local timeline to
the UTC timeline.

The `(hh, mm, ss)`

components are understood and constrainted as
follows:

`hh`

is the hour from`0`

to`23`

.`mm`

is the minute from`0`

to`59`

.`ss`

is the seconds from`0`

to`60`

.`60`

may happen whenever a leap second is added.

A `time`

value is said to be *valid* iff the values `(hh, mm, ss)`

are in the ranges mentioned above.

`of_date_time dt`

is the POSIX timestamp corresponding to
date-time `dt`

or `None`

if `dt`

has an invalid date,
invalid time or the date-time is not in the range
[min;max].

**Leap seconds.** Any date-time with a seconds value of `60`

, hence
representing a leap second addition, is mapped to the date-time
that happens 1 second later. Any date-time with a seconds value of
`59`

is mapped to the POSIX timestamp that represents this
instant, if a leap second was subtracted at that point, this is
the POSIX timestamp that represents this inexisting instant. See
the basics.

`val to_date_time : ?tz_offset_s:tz_offset_s -> t -> date * time`

`to_date_time ~tz_offset_s t`

is the date-time of the timestamp `t`

.

`tz_offset_s`

hints the time zone offset used for the resulting
daytime component (defaults to `0`

, i.e. UTC). The offset is not
honoured and fallbacks to `0`

in case the resulting date-time
rendering of the timestamp would yield an invalid
date. This means that you should always interpret the resulting
time component with the time zone offset it is paired with in the
result and not assume it will be the one you gave to the
function. Note that for real-world time zone offsets the fallback
to `0`

will only happen around Ptime.min and Ptime.max.
Formally the fallback occurs whenever ```
add_span t (Span.of_int_s
tz_offset_s)
```

is `None`

.

**Leap seconds.** No POSIX timestamp can represent a date-time
with a leap second added, hence this function will never return a
date-time with a `60`

seconds value. This function does return
inexisting UTC date-times with `59`

seconds whenever a leap second is
subtracted since POSIX timestamps do represent them. See the
basics.

**Subsecond precision.** POSIX timestamps with subsecond precision
are floored, i.e. the date-time always has the second mentioned in
the timestamp.

`val weekday : ?tz_offset_s:tz_offset_s -> t -> [ `Mon | `Tue | `Wed | `Thu | `Fri | `Sat | `Sun ]`

`weekday ~tz_offset_s t`

is the day in the 7-day week of timestamp `t`

expressed in the time zone offset `ts_offset_s`

(defaults to `0`

).

This can be used with the time zone offset result of to_date_time to convert timestamps to denormalized timestamp formats.

`val pp_rfc3339_error : Format.formatter -> rfc3339_error -> unit`

`pp_rfc3339_error ppf e`

prints an unspecified representation of
`e`

on `ppf`

.

`val rfc3339_error_to_msg : ('a, [ `RFC3339 of error_range * rfc3339_error ]) Result.result -> ('a, [> `Msg of string ]) Result.result`

`rfc3339_error_to_msg r`

converts RFC 3339 parse errors to error
messages.

`val of_rfc3339 : ?strict:bool -> ?sub:bool -> ?start:int -> string -> (t * tz_offset_s option * int, [> `RFC3339 of error_range * rfc3339_error ]) Result.result`

`of_rfc3339 ~strict ~sub ~start s`

parses an RFC 3339
`date-time`

starting at `start`

(defaults to `0`

) in `s`

to a triple `(t, tz, count)`

with:

`t`

the POSIX timestamp (hence on the UTC timeline).`tz`

, the optional time zone offset found in the timestamp.`None`

is returned iff the date-time satisfies the unknown local offset convention.`count`

the number of bytes read starting at`start`

to parse the timestamp. If`sub`

is`false`

(default) this is always`String.length s - start`

and`Error `Trailing_input`

is returned if there are still bytes in`s`

after the date-time was parsed. Use`~sub:true`

for allowing trailing input to exist.

If `strict`

is `true`

(defaults to `false`

) the parsing function
errors on timestamps with lowercase `'T'`

or `'Z'`

characters or
space separated date and times.

**Notes and limitations.**

- If
`start`

is not an index of`s`

,`Error ((start, start), `Eoi)`

is returned. - RFC 3339 allows a few degenerate (I say) timestamps with
non-zero time zone offsets to be parsed at the boundaries that
correspond to timestamps that cannot be expressed in UTC in RFC
3339 itself (e.g.
`0000-01-01T00:00:00+00:01`

). The function errors on these timestamps with``Invalid_stamp`

as they cannot be represented in the range [min;max]. - Leap seconds are allowed on any date-time and handled as in of_date_time
- Fractional parts beyond the picosecond (
`1e-12`

) are truncated.

`val to_rfc3339 : ?space:bool -> ?frac_s:int -> ?tz_offset_s:tz_offset_s -> t -> string`

`to_rfc3339_tz ~space ~frac_s ~tz_offset_s t`

formats the timestamp
`t`

according to a RFC 3339
`date-time`

production with:

`tz_offset_s`

hints the time zone offset to use, use`0`

for UTC. The hint is ignored in the following cases: if`tz_offset_s`

is not an integral number of minutes and its magnitude not in the range permitted by the standard, if`add_span t (Span.of_int_s tz_offset_s)`

is`None`

(the resulting timestamp rendering would not be RFC 3339 compliant). If either the hint is ignored or`tz_offset_s`

is unspecified then the unknown local offset convention is used to render the time zone component.`frac_s`

, clipped to the range [`0`

;`12`

] specifies that exactly`frac_s`

decimal digits of the fractional second of`t`

are rendered (defaults to`0`

).`space`

if`true`

the date and time separator is a space rather than a`'T'`

(not recommended but may be allowed by the protocol you are dealing with, defaults to`false`

).

`val pp_rfc3339 : ?space:bool -> ?frac_s:int -> ?tz_offset_s:tz_offset_s -> unit -> Format.formatter -> t -> unit`

`pp_rfc3339 ?space ?frac_s ?tz_offset_s () ppf t`

is
`Format.fprintf ppf "%s" (to_rfc3339 ?space ?frac_s ?tz_offset_s t)`

.

`val pp_human : ?frac_s:int -> ?tz_offset_s:tz_offset_s -> unit -> Format.formatter -> t -> unit`

`pp_human ~frac_s ~tz_offset_s () ppf t`

prints an unspecified, human
readable, locale-independent, representation of `t`

with:

`tz_offset_s`

hints the time zone offset to use. The hint is ignored in the following cases: if`tz_offset_s`

is not an integral number of minutes and its magnitude not in the range permitted by the standard, if`add_span t (Span.of_int_s tz_offset_s)`

is`None`

. If either the hint is ignored or`tz_offset_s`

is unspecified then RFC 3339's unknown local offset convention is used to render the time zone component.`frac_s`

clipped to the range [`0`

;`12`

] specifies that exactly`frac_s`

decimal digits of the fractional second of`t`

are rendered (defaults to`0`

).

**Note.** The output of this function is similar to but **not**
compliant with RFC 3339, it should only be used for presentation,
not as a serialization format.

`val dump : Format.formatter -> t -> unit`

`dump ppf t`

prints an unspecified raw representation of `t`

on `ppf`

.

POSIX time counts POSIX seconds since the epoch
1970-01-01 00:00:00 UTC. As such a POSIX timestamp is **always**
on the UTC timeline.

POSIX time doesn't count leap seconds, so by definition it cannot represent them. One way of viewing this is that whenever a leap second is added a POSIX second lasts two SI seconds and whenever a leap second is subtracted a POSIX second lasts zero SI second.

Ptime does not provide any mean to convert the duration between two POSIX timestamps to SI seconds. The reason is that in order to accurately find this number, a leap second table is needed. However since this table may change every six months, Ptime decides not to include it so as not to potentially become incorrect every six months.

This decision has the following implications. First it should be
realised that the durations mentioned by the add_span,
sub_span and diff functions are expressed in *POSIX seconds* which may represent zero, one, or two SI
seconds. For example if we add 1 second with
add_span to the POSIX timestamp for 1998-12-31 23:59:59 UTC,
what we get is the timestamp for 1999-01-01 00:00:00 UTC:

```
let get = function None -> assert false | Some v -> v
let utc d t = get @@ Ptime.of_date_time (d, (t, 0))
let t0 = utc (1998, 12, 31) (23, 59, 59)
let t1 = utc (1999, 01, 01) (00, 00, 00)
let one_s = Ptime.Span.of_int_s 1
let () = assert (Ptime.equal (get @@ Ptime.add_span t0 one_s) t1)
```

However since the leap second 1998-12-31 23:59:60 UTC exists,
*two* actual SI seconds elapsed between `t0`

and `t1`

. Now if we use
diff to find the POSIX duration that elapsed between
`t0`

and `t1`

we get one POSIX second:

```
let () = assert (Ptime.Span.equal (Ptime.diff t1 t0) one_s)
```

But still, two SI seconds elapsed between these two points in
time. Note also that no value of type t can represent the UTC
timetamp 1998-12-31 23:59:60 and hence Ptime.to_date_time
will never return a date-time with a seconds value of `60`

. In
fact both 1998-12-31 23:59:60 UTC and 1999-01-01 00:00:00 UTC are
represented by the same timestamp:

```
let t2 = utc (1998, 12, 31) (23, 59, 60)
let () = assert (Ptime.equal t1 t2)
```

This is true of any added leap second, we map it on the first second of the next minute, thus matching the behaviour of POSIX's mktime function.

If a leap second is subtracted on a day the following occurs – 2015, as of writing this never happened. Let YYYY-06-30 23:59:58 be the instant a leap second is subtracted, this means that the next UTC date-time, one SI second later, is YYYY-07-01 00:00:00. However if we diff the two instants:

```
let y = 9999 (* hypothetical year were this happens *)
let t0 = utc (y, 06, 30) (23, 59, 58)
let t1 = utc (y, 07, 01) (00, 00, 00)
let two_s = Ptime.Span.of_int_s 2
let () = assert (Ptime.Span.equal (Ptime.diff t1 t0) two_s)
```

We get two POSIX seconds, but only one SI second elapsed between these two points in time. It should also be noted that POSIX time will represent a point that never existed in time namely YYYY-06-30 23:59:59, the POSIX second with 0 SI second duration and that Ptime.to_date_time will return a date-time value for this timestamp even though it never existed:

```
let t2 = utc (y, 06, 30) (23, 59, 59)
let () = assert (Ptime.equal (get @@ Ptime.add_span t0 one_s) t2)
```

The following points should be taken into account

- Ptime is not a calendar library and will never be.
- Ptime can only represent picosecond precision timestamps in
the range [Ptime.min;Ptime.max]. It is however able to
convert
*any*of these timestamps to a valid date-time or RFC 3339 timestamp. POSIX time in general is ill-suited to measure wall-clock time spans for the following reasons.

- POSIX time counts time in POSIX seconds. POSIX
seconds can represent 2, 1 or 0 SI seconds.
`Ptime`

offers no mechanism to determine the SI duration between two timestamps, see the basics. - The POSIX timestamps returned by your platform are not monotonic: they are subject to operating system time adjustements and can even go back in time. If you need to measure time spans in a single program run use a monotonic time source (e.g. Mtime).

- POSIX time counts time in POSIX seconds. POSIX
seconds can represent 2, 1 or 0 SI seconds.