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DateTimeParser

Hex.pm Version Hex docs License: MIT Contributor Covenant

DateTimeParser is a tokenizer for strings that attempts to parse into a DateTime, NaiveDateTime if timezone is not determined, Date, or Time.

You're currently looking at the master branch. Check out the docs for the latest published version.

Documentation

See examples automatically generated by the tests

The biggest ambiguity between datetime formats is whether it's ymd (year month day), mdy (month day year), or dmy (day month year); this is resolved by checking if there are slashes or dashes. If slashes, then it will try dmy first. All other cases will use the international format ymd. Sometimes, if the conditions are right, it can even parse dmy with dashes if the month is a vocal month (eg, "Jan").

If the string consists of only numbers, then we will try two other parsers depending on the number of digits: Epoch or Serial. Otherwise, we'll try the tokenizer.

If the string is 10-11 digits with optional precision, then we'll try to parse it as a Unix Epoch timestamp.

If the string is 1-5 digits with optional precision, then we'll try to parse it as a Serial timestamp (spreadsheet time) treating 1899-12-31 as 1. This will cause Excel-produced dates from 1900-01-01 until 1900-03-01 to be incorrect, as they really are.

digits parser range notes
1-5 Serial low = 1900-01-01, high = 2173-10-15. Negative numbers go to 1626-03-17 Floats indicate time. Integers do not.
6-9 Tokenizer any This allows for "20190429" to be parsed as 2019-04-29
10-11 Epoch low = -1100-02-15 14:13:21, high = 5138-11-16 09:46:39 If padded with 0s, then it can capture entire range.
else Tokenizer any

Required reading

Examples

iex> DateTimeParser.parse("19 September 2018 08:15:22 AM")
{:ok, ~N[2018-09-19 08:15:22]}

iex> DateTimeParser.parse_datetime("19 September 2018 08:15:22 AM")
{:ok, ~N[2018-09-19 08:15:22]}

iex> DateTimeParser.parse_datetime("2034-01-13", assume_time: true)
{:ok, ~N[2034-01-13 00:00:00]}

iex> DateTimeParser.parse_datetime("2034-01-13", assume_time: ~T[06:00:00])
{:ok, ~N[2034-01-13 06:00:00]}

iex> DateTimeParser.parse("invalid date 10:30pm")
{:ok, ~T[22:30:00]}

iex> DateTimeParser.parse("2019-03-11T99:99:99")
{:ok, ~D[2019-03-11]}

iex> DateTimeParser.parse("2019-03-11T10:30:00pm UNK")
{:ok, ~N[2019-03-11T22:30:00]}

iex> DateTimeParser.parse("2019-03-11T22:30:00.234+00:00")
{:ok, DateTime.from_naive!(~N[2019-03-11T22:30:00.234Z], "Etc/UTC")}
# `~U[2019-03-11T22:30:00.234Z]` in Elixir 1.9+

iex> DateTimeParser.parse_date("2034-01-13")
{:ok, ~D[2034-01-13]}

iex> DateTimeParser.parse_date("01/01/2017")
{:ok, ~D[2017-01-01]}

iex> DateTimeParser.parse_datetime("1564154204")
{:ok, DateTime.from_naive!(~N[2019-07-26T15:16:44Z], "Etc/UTC")}
# `~U[2019-07-26T15:16:44Z]` in Elixir 1.9+

iex> DateTimeParser.parse_datetime("41261.6013888889")
{:ok, ~N[2012-12-18T14:26:00]}

iex> DateTimeParser.parse_date("44262")
{:ok, ~D[2021-03-07]}
# This is a serial number date, commonly found in spreadsheets, eg: `=VALUE("03/07/2021")`

iex> DateTimeParser.parse_datetime("1/1/18 3:24 PM")
{:ok, ~N[2018-01-01T15:24:00]}

iex> DateTimeParser.parse_datetime("1/1/18 3:24 PM", assume_utc: true)
{:ok, DateTime.from_naive!(~N[2018-01-01T15:24:00Z], "Etc/UTC")}
# `~U[2018-01-01T15:24:00Z]` in Elixir 1.9+

iex> DateTimeParser.parse_datetime(~s|"Mar 28, 2018 7:39:53 AM PDT"|, to_utc: true)
{:ok, DateTime.from_naive!(~N[2018-03-28T14:39:53Z], "Etc/UTC")}

iex> {:ok, datetime} = DateTimeParser.parse_datetime(~s|"Mar 1, 2018 7:39:53 AM PST"|)
iex> datetime
#DateTime<2018-03-01 07:39:53-08:00 PST PST8PDT>

iex> DateTimeParser.parse_datetime(~s|"Mar 1, 2018 7:39:53 AM PST"|, to_utc: true)
{:ok, DateTime.from_naive!(~N[2018-03-01T15:39:53Z], "Etc/UTC")}

iex> {:ok, datetime} = DateTimeParser.parse_datetime(~s|"Mar 28, 2018 7:39:53 AM PDT"|)
iex> datetime
#DateTime<2018-03-28 07:39:53-07:00 PDT PST8PDT>

iex> DateTimeParser.parse_time("10:13pm")
{:ok, ~T[22:13:00]}

iex> DateTimeParser.parse_time("10:13:34")
{:ok, ~T[10:13:34]}

iex> DateTimeParser.parse_time("18:14:21.145851000000Z")
{:ok, ~T[18:14:21.145851]}

iex> DateTimeParser.parse_datetime(nil)
{:error, "Could not parse nil"}

Installation

Add date_time_parser to your list of dependencies in mix.exs:

def deps do
  [
    {:date_time_parser, "~> 1.1.2"}
  ]
end

Configuration

# This is the default config
alias DateTimeParser.Parser
config :date_time_parser, parsers: [Parser.Epoch, Parser.Serial, Parser.Tokenizer]

# To enable only specific parsers, include them in the :parsers key.
config :date_time_parser, parsers: [Parser.Tokenizer]

# Or in runtime, pass in the parsers in the function.
DateTimeParser.parse(mystring, parsers: [Parser.Tokenizer])

Write your own parser

You can write your own parser!

If the built-in parsers are not applicable for your use-case, you may build your own parser to use with this library. Let's write a simple one together.

First I will check DateTimeParser.Parser to see what behaviour my new parser should implement. It needs two functions:

  1. c:DateTimeParser.Parser.preflight/1
  2. c:DateTimeParser.Parser.parse/1

These functions accept the t:DateTimeParser.Parser.t/0 struct which contains the options supplied by the user, the string itself, and the context for which you should return your result. For example, if the context is :time then you should return a %Time{}; if :datetime you should return either a %NaiveDateTime{} or a %DateTime{}; if :date then you should return a %Date{}.

Let's implement a parser that reads a special time string. Our string will represent time, but all the digits are shifted up by 10 and must be prefixed with the secret word: "boomshakalaka:". For example, the real world time of 01:10 is represented as boomshakalaka:11:20 in our toy time format. 12:30 is represented as boomshakalaka:22:40, and 5:55 is represented as boomshakalaka:15:65.

defmodule MyParser do
  @behaviour DateTimeParser.Parser
  @secret_regex ~r|boomshakalaka:(?<time>\d{2}:\d{2})|

  def preflight(%{string: string} = parser) do
    case Regex.named_captures(@secret_regex, string) do
      %{"time" => time} ->
        {:ok, %{parser | preflight: time}}

      nil ->
        {:error, :not_compatible}
    end
  end

  # ... more below
end

We'll stop here first and go through the preflight function. Our special parser will only be attempted if the supplied string has any named captures from the regex. That is, it must begin with bookshakalaka: followed by 2 digits, a colon, and 2 more digits. These digits are extracted out like 00:00 where 0 is any digit. If 05:40 is passed in, it would not be compatible so the parser will be skipped.

Now let's parse the time:

def parse(%{preflight: time} = parser) do
  [hour, minute] = String.split(time, ":")
  {hour, ""} = Integer.parse(hour)
  {minute, ""} = Integer.parse(minute)
  result = Time.new(hour - 10, minute - 10, 0, {0, 0})
  for_context(parser.context, result)
end

defp for_context(:datetime, _result), do: :error
defp for_context(:date, _result), do: :error
defp for_context(:time, result), do: result

Notice that we need to consider context of the result. If the user asked for a DateTime, then we need to give them one. In our toy format, it only represents time, so therefore we must return an error when the context is a :datetime or :date.

DateTimeParser.parse_time("boomshakalaka:11:11", parsers: [MyParser])
#=> {:ok, ~T[01:01:00]}

DateTimeParser.parse_date("boomshakalaka:11:11", parsers: [MyParser])
#=> {:error, "Could not parse \"boomshakalaka:11:11\""}

DateTimeParser.parse_datetime("boomshakalaka:11:11", parsers: [MyParser])
#=> {:error, "Could not parse \"boomshakalaka:11:11\""}

DateTimeParser.parse("boomshakalaka:11:11", parsers: [MyParser])
#=> {:ok, ~T[01:01:00]}

Should I use this library?

Only as a last resort. Parsing dates from strings is educated guessing at best. Since Elixir natively supports ISO-8601 parsing (see from_iso8601/2 functions), it's highly recommended that you rely on that first and foremost.

When designing your API that involves dates and strings, be specific with your requirements and supported DateTime strings, and preferably only support ISO-8601 with no exceptions. There is no ambiguity with this format so parsing to DateTime (or Date or Time) will always be correct.

This library is helpful when you must accept ambiguous DateTime string formats and having incorrect results is acceptable. Do not use this library when the resulting (and possibly incorrect) DateTime has catastrophic and dangerous effects in your system.

How to store future timestamps

see guide

tldr: rules change, so don't convert to UTC too early. The future might change the timezone conversion rules.

Changelog

View Changelog

Upgrading from 0.x to 1.0

  • If you use parse_datetime/1, then change to parse_datetime/2 with the second argument as a keyword list to assume_time: true and to_utc: true. In 0.x, it would merge ~T[00:00:00] if the time tokens could not be parsed; in 1.x, you have to opt into this behavior. Also in 0.x, a non-UTC timezone would automatically convert to UTC; in 1.x, the original timezone will be kept instead.
  • If you use parse_date/1, then change to parse_date/2 with the second argument as a keyword list to assume_date: true. In 0.x, it would merge Date.utc_today() with the found date tokens; in 1.x, you need to opt into this behavior.
  • If you use parse_time, there is no breaking change but parsing has been improved.
  • Not a breaking change, but 1.x introduces parse/2 that will return the best struct from the tokens. This may influence your usage.

Contributing

How to contribute

Special Thanks

TaxJar