Elixir Cheatsheet

Dates and Times

Use this Elixir reference while you build software engineering projects, review code, or refresh the syntax you reach for most.

The Four Calendar Types

Type	Holds	Sigil
`Date`	year-month-day	`~D[2026-01-15]`
`Time`	hour:minute:second(.µs)	`~T[13:30:00]`
`NaiveDateTime`	date + time, no time zone	`~N[2026-01-15 13:30:00]`
`DateTime`	date + time + time zone	`~U[2026-01-15 13:30:00Z]`

Use DateTime in UTC for timestamps; NaiveDateTime only when the zone is genuinely unknown or irrelevant.

Now and Today

Date.utc_today()                    # ~D[2026-07-05]
Time.utc_now()
DateTime.utc_now()                  # ~U[2026-07-05 18:04:12.345678Z]
NaiveDateTime.utc_now()
System.os_time(:millisecond)        # integer Unix time
System.monotonic_time(:millisecond) # for measuring durations, never wall clock

Building and Parsing

Date.new(2026, 1, 15)                       # {:ok, ~D[2026-01-15]}
Date.new!(2026, 1, 15)
DateTime.new!(~D[2026-01-15], ~T[13:30:00], "Etc/UTC")

Date.from_iso8601("2026-01-15")             # {:ok, ~D[2026-01-15]}
DateTime.from_iso8601("2026-01-15T13:30:00Z")  # {:ok, dt, utc_offset}
NaiveDateTime.from_iso8601!("2026-01-15 13:30:00")

Date.to_iso8601(date)                       # "2026-01-15"
DateTime.to_iso8601(dt)                     # "2026-01-15T13:30:00Z"
to_string(date)                             # ISO 8601 via String.Chars

DateTime.from_unix!(1_768_500_000)          # Unix seconds -> DateTime
DateTime.to_unix(dt)

The standard library speaks ISO 8601 only. For "Jan 15, 2026"-style parsing/formatting, use Calendar.strftime/3 (formatting, built in) or a library like timex.

Calendar.strftime(dt, "%Y-%m-%d %H:%M")     # "2026-01-15 13:30"
Calendar.strftime(dt, "%A, %B %d")          # "Thursday, January 15"

Arithmetic and Comparison

Date.add(~D[2026-01-15], 30)                # ~D[2026-02-14]
Date.diff(~D[2026-02-14], ~D[2026-01-15])   # 30 (days)
DateTime.add(dt, 3600, :second)             # also :minute, :hour, :day
DateTime.diff(dt2, dt1)                     # seconds; DateTime.diff(dt2, dt1, :millisecond)

Date.compare(d1, d2)                        # :lt | :eq | :gt
DateTime.compare(dt1, dt2)
DateTime.before?(dt1, dt2); DateTime.after?(dt1, dt2)

Never compare DateTime structs with </> — structural comparison is wrong for dates; always use compare/2 (this also applies to Enum.sort/2: Enum.sort(dates, Date) passes the module as the comparator).

Date Components and Ranges

date.year; date.month; date.day
Date.day_of_week(~D[2026-07-05])            # 7 (Monday = 1)
Date.beginning_of_month(date); Date.end_of_month(date)
Date.beginning_of_week(date)
Date.days_in_month(date)
Date.leap_year?(date)

Date.range(~D[2026-01-01], ~D[2026-01-31])  # enumerable range of dates
|> Enum.filter(&(Date.day_of_week(&1) == 1))

Time Zones

# mix.exs: {:tzdata, "~> 1.1"}
# config: config :elixir, :time_zone_database, Tzdata.TimeZoneDatabase

DateTime.now!("America/Los_Angeles")
DateTime.shift_zone!(utc_dt, "Europe/Berlin")

Only Etc/UTC works out of the box; any other zone needs a time zone database (usually tzdata). Standard practice: store and compute in UTC, shift to the user's zone at the display edge.

Measuring Elapsed Time

t0 = System.monotonic_time(:millisecond)
work()
elapsed_ms = System.monotonic_time(:millisecond) - t0

{micros, result} = :timer.tc(fn -> work() end)