DateTime

In Ruby consider the DateTime class. It fetches the current date and relative dates. We use operators to adjust days and months, and we can even check for leap years.

By using methods like DateTime.now, we can get the current time. Meanwhile, properties like month, year, and day can reveal date info.

This Ruby program gets the current date with DateTime.now. It also shows how to access the month, day and year parts of the DateTime.

require "date"

# Get current date.
current = DateTime.now

# Print current month, day and year.
print "Month: ", current.month, "\n"
print "  Day: ", current.day, "\n"
print " Year: ", current.year, "\n"
Month: 2
  Day: 21
 Year: 2017

Add, subtract

We can add or subtract an integer from a Date. Subtracting one yields the previous day, and adding one moves to the next day. In this way, we compute yesterday and tomorrow.

require "date"

# First day of 2014.
d = Date.new(2014, 1, 1)

# Subtract one day to get the last day of 2013.
subtract = d - 1

# Add one day to get the next day.
add = d + 1

# Display values.
puts subtract
puts d
puts add
2013-12-31
2014-01-01
2014-01-02

Gm, local

We create arbitrary times with the Time.gm and Time.local methods. Time.gm returns a UTC time. Time.local returns a time adjusted for your local time zone.

# Create a UTC time for the specified values.
t = Time.gm(2013, "mar", 15)
puts t

# Create time with local time zone.
t = Time.local(2010, "apr", 10)
puts t

# More arguments can be passed.
t = Time.gm(2012, "jun", 30, 6, 30, 30, 50)
puts t
2013-03-15 00:00:00 UTC
2010-04-10 00:00:00 -0700
2012-06-30 06:30:30 UTC

Leap

This method tells us if a year is a leap year or not. Leap years have an extra day. We must first create a Date object with the Date.new initialize method.

require "date"

def days_in_year(year)
    d = Date.new(year, 1, 1)
    # See if the year is a leap year.
    if d.leap?
        return 366
    else
        return 365
    end
end

# Test years for leap status.
2000.upto(2010) do |year|
    puts year.to_s << " has " << days_in_year(year).to_s << " days"
end
2000 has 366 days
2001 has 365 days
2002 has 365 days
2003 has 365 days
2004 has 366 days
2005 has 365 days
2006 has 365 days
2007 has 365 days
2008 has 366 days
2009 has 365 days
2010 has 365 days

Upto, downto

Dates have helpful iterators. In Ruby we often prefer iterators over loops. For upto, the initial date is incremented upwards, a day at a time. And downto decrements the day.

require "date"

d1 = Date.new(2014, 10, 5)
d2 = Date.new(2014, 10, 7)

puts "upto"

# Iterate from low to high date.
d1.upto(d2) do |d|
    puts d
end

puts "downto"

# Iterate from high to low date.
d2.downto(d1) do |d|
    puts d
end
upto
2014-10-05
2014-10-06
2014-10-07
downto
2014-10-07
2014-10-06
2014-10-05

Step

This iterator is more flexible than upto or downto. We specify the "increment" or decrement value. So a step of 7 advances an entire week on each iteration.

require "date"

first = Date.new(2014, 10, 1)
last = Date.new(2014, 11, 1)

# Advance seven days each iteration (these are Wednesdays).
first.step(last, 7) do |d|
    puts d
end
2014-10-01
2014-10-08
2014-10-15
2014-10-22
2014-10-29

Next

The next and succ methods return the successive date—the "tomorrow" date. To me, the next() method is more clearly named. In this program, I specify the end date of October.

require "date"

d = Date.new(2014, 10, 31)

# Get the next date with succ or next.
d2 = d.succ
puts d2

d2 = d.next
puts d2
2014-11-01
2014-11-01

Months

We use shift operators with two greater than (or less than) signs to add (or subtract) months. This changes the month in a date. The plus and minus are used for days.

require "date"

d = Date.new(2015, 5, 1)

# Add two months to the date.
d2 = d >> 2
puts d2

# Subtract three months from the date.
d3 = d << 3
puts d3
2015-07-01
2015-02-01

Wday

This number indicates the "day of week" or weekday of a date. It returns the numbers 0 through 6, with 0 meaning Sunday and 6 meaning Saturday.

require "date"

# This is a Sunday, so its number is 0.
d = Date.new(2014, 10, 5)
puts d.wday

# Next day is a Monday (number 1).
d = d.next
puts d.wday

# Tuesday (number 2).
d = d.next
puts d.wday

# One week later is Tuesday again.
d += 7
puts d.wday
0
1
2
2

With DateTime in Ruby, we access many date functions. These would be hard to implement. It is typically best to reuse these library methods where possible.