# Simple-date reference manual

Simple-date provides types (CLOS classes) for dates, timestamps, and intervals similar to the ones SQL databases use, in order to be able to store and read these to and from a database in a straighforward way. A few obvious operations are defined on these types.

The most glaring defect of this library is its ignorance of time zones. It pretends the whole world lives in UTC. Use with care.

When this libary is loaded after CL-postgres, it will register suitable SQL readers and writers for the associated database types.

## Contents

1. Date type
2. Timestamp type
3. Interval type
4. Operations
5. Symbol-index

## Date type

class date

Represents a date, with no time-of-day information.

function encode-date (year month day)
→ date

Creates a date object.

function decode-date (date)
→ (values year month day)

Extract the elements from a date object.

function day-of-week (date)
→ integer

Determine the day of the week that the given date falls on. Value ranges from 0 to 6, with 0 being Sunday and 6 being Saturday.

## Timestamp type

class timestamp

Represents an absolute timestamp, with a millisecond precision.

function encode-timestamp (year month day &optional (hour 0) (minute 0) (second 0) (millisecond 0))
→ timestamp

Create a timestamp. No negative values or values outside of an arguments normal range (i.e. 60 for minutes, 1000 for milliseconds) should be passed.

function decode-timestamp (timestamp)
→ (values year month day hour minute second millisecond)

Decode a timestamp into its components.

function timestamp-to-universal-time (timestamp)
→ universal-time

Convert a timestamp to the corresponding universal-time, rounding to seconds. Note that this will treat the timestamp as if it were in UTC.

function universal-time-to-timestamp (universal-time)
→ timestamp

Create a timestamp from a universal time. Again, the resulting timestamp should be treated as if it were in UTC.

## Interval type

class interval

An interval represents a period of time. It contains both an absolute part in milliseconds (days, weeks, minutes, etc are always the same length), and a relative part for months and years ― the amount of time that a month or year represents is not always the same.

function encode-interval (&key (year 0) (month 0) (week 0) (day 0) (hour 0) (minute 0) (second 0) (millisecond 0))
→ interval

Create an interval. Arguments may be negative and of any size.

function decode-interval (interval)
→ (values year month day hour minute second millisecond)

Decompose an interval into parts. Note that these may be different from the parameters that created it ― an interval of 3600 seconds is the same as one of 1 hour.

## Operations

To prevent a proliferation of different function names, generic functions are used for operations on time values. The semantics of these differ for the type of the operands.

method time-add (a b)
→ value

Adds two time-related objects. Adding an interval to a date or timestamp will return a new date or timestamp, increased by the value of the interval. Adding two intervals returns a new interval with the sum of the two arguments. Integers can be used in place of intervals, and will be interpreted as an amount of milliseconds.

method time-subtract (a b)
→ value

Subtracts time-related objects from each other. Subtracting two dates or timestamps results in an interval that represents the difference between them. Similarly, subtracting two intervals also gives their difference.

method time= (a b)
→ boolean

Compare two time-related values, returns a boolean indicating whether they denote the same time or period.

method time< (a b)
→ boolean

Compare two time-related values, returns a boolean indicating whether the first is less than the second.

method time> (a b)
→ boolean

Compare two time-related values, returns a boolean indicating whether the first is greater than the second.

function time<= (a b)
→ boolean

The inverse of `time>`.

function time>= (a b)
→ boolean

The inverse of `time<`.

## Symbol-index

- date
- day-of-week
- decode-date
- decode-interval
- decode-timestamp
- encode-date
- encode-interval
- encode-timestamp
- interval
- time>
- time<
- time>=
- time<=
- time-add
- time-subtract
- time=
- timestamp
- timestamp-to-universal-time
- universal-time-to-timestamp