class Time

Practical examples and pitfalls for Time

Time examples: preserve an instant while changing its presentation

Practical notes by Ruby-Doc.org

Record the offset when the input supplies one

An event recorded at three in the afternoon with a one-hour positive UTC offset represents the same instant as two in the afternoon UTC. The hour printed on the clock changes when the representation changes; the event itself does not move. Use an explicit offset in a reproducible example instead of inheriting whichever timezone happens to be configured on the machine.

Example 1
require "time"
event = Time.new(2026, 9, 10, 15, 0, 0, "+01:00")
utc = event.getutc
puts utc.iso8601
p event == utc
p event.utc_offset
Expected output
2026-09-10T14:00:00Z
true
3600

A fixed offset does not identify a geographic timezone or supply its future daylight-saving rules. If an application schedules a recurring local appointment, it needs a timezone policy as well as a time value. Keep that scheduling requirement distinct from recording the instant at which a request arrived.

Subtract instants to obtain elapsed seconds

For two recorded events, subtraction gives an interval in seconds. A short batch report can then label that result explicitly, instead of leaving a bare number whose unit has to be guessed.

Example 2
started = Time.utc(2026, 9, 10, 14, 0, 0)
finished = Time.utc(2026, 9, 10, 14, 1, 35)
p finished - started
Expected output
95.0

If the report needs calendar days rather than elapsed seconds, use a date-based rule. A day on a local calendar is not always interchangeable with adding a fixed number of seconds across a clock change. For measuring how long a running operation takes, a monotonic clock is usually the relevant tool because wall-clock time can be adjusted.

Keep formatting out of the comparison

Compare time values before turning them into display strings. Two equal instants can have different printed offsets, and a human-readable format may omit precision or sort poorly as text. Choose a consistent serialization format for logs and document whether fractional seconds are retained.

The examples use fixed inputs so their results do not depend on the current date. In your tests, include an offset-bearing input and a boundary near midnight. That is where an otherwise plausible display conversion often reveals that the wrong day was shown.

API reference: Time API reference

Related: Date · Kernel

A Time object represents a date and time:

Time.new(2000, 1, 1, 0, 0, 0) # => 2000-01-01 00:00:00 -0600

Although its value can be expressed as a single numeric (see Epoch Seconds below), it can be convenient to deal with the value by parts:

t = Time.new(-2000, 1, 1, 0, 0, 0.0)
# => -2000-01-01 00:00:00 -0600
t.year # => -2000
t.month # => 1
t.mday # => 1
t.hour # => 0
t.min # => 0
t.sec # => 0
t.subsec # => 0

t = Time.new(2000, 12, 31, 23, 59, 59.5)
# => 2000-12-31 23:59:59.5 -0600
t.year # => 2000
t.month # => 12
t.mday # => 31
t.hour # => 23
t.min # => 59
t.sec # => 59
t.subsec # => (1/2)

Epoch Seconds

Epoch seconds is the exact number of seconds (including fractional subseconds) since the Unix Epoch, January 1, 1970.

You can retrieve that value exactly using method Time.to_r:

Time.at(0).to_r        # => (0/1)
Time.at(0.999999).to_r # => (9007190247541737/9007199254740992)

Other retrieval methods such as Time#to_i and Time#to_f may return a value that rounds or truncates subseconds.

Time Resolution

A Time object derived from the system clock (for example, by method Time.now) has the resolution supported by the system.

Time Internal Representation

Conceptually, Time class uses a rational value to represent the number of seconds from Epoch, 1970-01-01 00:00:00 UTC. There are no boundary or resolution limitations. The value can be obtained using Time#to_r.

The Time class always uses the Gregorian calendar. I.e. the proleptic Gregorian calendar is used. Other calendars, such as Julian calendar, are not supported.

The implementation uses a signed 63 bit integer, Integer (Bignum) object or Ratoinal object to represent a rational value. (The signed 63 bit integer is used regardless of 32 and 64 bit environments.) The value represents the number of nanoseconds from Epoch. The signed 63 bit integer can represent 1823-11-12 to 2116-02-20. When Integer or Rational object is used (before 1823, after 2116, under nanosecond), Time works slower than when the signed 63 bit integer is used.

Ruby uses the C function localtime and gmtime to map between the number and 6-tuple (year,month,day,hour,minute,second). localtime is used for local time and gmtime is used for UTC.

Integer and Rational has no range limit, but the localtime and gmtime has range limits due to the C types time_t and struct tm. If that limit is exceeded, Ruby extrapolates the localtime function.

time_t can represent 1901-12-14 to 2038-01-19 if it is 32 bit signed integer, -292277022657-01-27 to 292277026596-12-05 if it is 64 bit signed integer. However localtime on some platforms doesn’t supports negative time_t (before 1970).

struct tm has tm_year member to represent years. (tm_year = 0 means the year 1900.) It is defined as int in the C standard. tm_year can represent years between -2147481748 to 2147485547 if int is 32 bit.

Ruby supports leap seconds as far as if the C function localtime and gmtime supports it. They use the tz database in most Unix systems. The tz database has timezones which supports leap seconds. For example, “Asia/Tokyo” doesn’t support leap seconds but “right/Asia/Tokyo” supports leap seconds. So, Ruby supports leap seconds if the TZ environment variable is set to “right/Asia/Tokyo” in most Unix systems.

Examples

All of these examples were done using the EST timezone which is GMT-5.

Creating a New Time Instance

You can create a new instance of Time with Time.new. This will use the current system time. Time.now is an alias for this. You can also pass parts of the time to Time.new such as year, month, minute, etc. When you want to construct a time this way you must pass at least a year. If you pass the year with nothing else time will default to January 1 of that year at 00:00:00 with the current system timezone. Here are some examples:

Time.new(2002)         #=> 2002-01-01 00:00:00 -0500
Time.new(2002, 10)     #=> 2002-10-01 00:00:00 -0500
Time.new(2002, 10, 31) #=> 2002-10-31 00:00:00 -0500

You can pass a UTC offset:

Time.new(2002, 10, 31, 2, 2, 2, "+02:00") #=> 2002-10-31 02:02:02 +0200

Or a timezone object:

zone = timezone("Europe/Athens")      # Eastern European Time, UTC+2
Time.new(2002, 10, 31, 2, 2, 2, zone) #=> 2002-10-31 02:02:02 +0200

You can also use Time.local and Time.utc to infer local and UTC timezones instead of using the current system setting.

You can also create a new time using Time.at which takes the number of seconds (with subsecond) since the Unix Epoch.

Time.at(628232400) #=> 1989-11-28 00:00:00 -0500

Working with an Instance of Time

Once you have an instance of Time there is a multitude of things you can do with it. Below are some examples. For all of the following examples, we will work on the assumption that you have done the following:

t = Time.new(1993, 02, 24, 12, 0, 0, "+09:00")

Was that a monday?

t.monday? #=> false

What year was that again?

t.year #=> 1993

Was it daylight savings at the time?

t.dst? #=> false

What’s the day a year later?

t + (60*60*24*365) #=> 1994-02-24 12:00:00 +0900

How many seconds was that since the Unix Epoch?

t.to_i #=> 730522800

You can also do standard functions like compare two times.

t1 = Time.new(2010)
t2 = Time.new(2011)

t1 == t2 #=> false
t1 == t1 #=> true
t1 <  t2 #=> true
t1 >  t2 #=> false

Time.new(2010,10,31).between?(t1, t2) #=> true

What’s Here

First, what’s elsewhere. Class Time:

Here, class Time provides methods that are useful for:

Methods for Creating

Methods for Fetching

Methods for Querying

Methods for Comparing

Methods for Converting

Methods for Rounding

For the forms of argument zone, see Timezone Specifiers.

Timezone Specifiers

Certain Time methods accept arguments that specify timezones:

The value given with any of these must be one of the following (each detailed below):

Hours/Minutes Offsets

The zone value may be a string offset from UTC in the form '+HH:MM' or '-HH:MM', where:

Examples:

t = Time.utc(2000, 1, 1, 20, 15, 1) # => 2000-01-01 20:15:01 UTC
Time.at(t, in: '-23:59')            # => 1999-12-31 20:16:01 -2359
Time.at(t, in: '+23:59')            # => 2000-01-02 20:14:01 +2359

Single-Letter Offsets

The zone value may be a letter in the range 'A'..'I' or 'K'..'Z'; see List of military time zones:

t = Time.utc(2000, 1, 1, 20, 15, 1) # => 2000-01-01 20:15:01 UTC
Time.at(t, in: 'A')                 # => 2000-01-01 21:15:01 +0100
Time.at(t, in: 'I')                 # => 2000-01-02 05:15:01 +0900
Time.at(t, in: 'K')                 # => 2000-01-02 06:15:01 +1000
Time.at(t, in: 'Y')                 # => 2000-01-01 08:15:01 -1200
Time.at(t, in: 'Z')                 # => 2000-01-01 20:15:01 UTC

Integer Offsets

The zone value may be an integer number of seconds in the range -86399..86399:

t = Time.utc(2000, 1, 1, 20, 15, 1) # => 2000-01-01 20:15:01 UTC
Time.at(t, in: -86399)              # => 1999-12-31 20:15:02 -235959
Time.at(t, in: 86399)               # => 2000-01-02 20:15:00 +235959

Timezone Objects

The zone value may be an object responding to certain timezone methods, an instance of Timezone and TZInfo for example.

The timezone methods are:

A custom timezone class may have these instance methods, which will be called if defined:

Time-Like Objects

A Time-like object is a container object capable of interfacing with timezone libraries for timezone conversion.

The argument to the timezone conversion methods above will have attributes similar to Time, except that timezone related attributes are meaningless.

The objects returned by local_to_utc and utc_to_local methods of the timezone object may be of the same class as their arguments, of arbitrary object classes, or of class Integer.

For a returned class other than Integer, the class must have the following methods:

For a returned Integer, its components, decomposed in UTC, are interpreted as times in the specified timezone.

Timezone Names

If the class (the receiver of class methods, or the class of the receiver of instance methods) has find_timezone singleton method, this method is called to achieve the corresponding timezone object from a timezone name.

For example, using Timezone:

class TimeWithTimezone < Time
  require 'timezone'
  def self.find_timezone(z) = Timezone[z]
end

TimeWithTimezone.now(in: "America/New_York")        #=> 2023-12-25 00:00:00 -0500
TimeWithTimezone.new("2023-12-25 America/New_York") #=> 2023-12-25 00:00:00 -0500

Or, using TZInfo:

class TimeWithTZInfo < Time
  require 'tzinfo'
  def self.find_timezone(z) = TZInfo::Timezone.get(z)
end

TimeWithTZInfo.now(in: "America/New_York")          #=> 2023-12-25 00:00:00 -0500
TimeWithTZInfo.new("2023-12-25 America/New_York")   #=> 2023-12-25 00:00:00 -0500

You can define this method per subclasses, or on the toplevel Time class.