Cookbooks
Ruby
Chapter 7. Dates and Times
Calculating a date in the future or past

Ruby:
Calculating a date in the future or past

How to:

Ruby makes playing with dates a breeze using its built-in Date class and the active_support gem for some extra sugar. Here’s how it’s done:

require 'date'
require 'active_support/core_ext/integer'

# Get today's date
today = Date.today
puts "Today is: #{today}"

# Calculate a date 10 days in the future
future_date = today + 10
puts "10 days from now will be: #{future_date}"

# Calculate a date 30 days in the past
past_date = today - 30
puts "30 days ago it was: #{past_date}"

# More complex calculations with active_support
puts "In 2 months, it will be: #{2.months.from_now.to_date}"
puts "100 days ago, it was: #{100.days.ago.to_date}"

Sample output:

Today is: 2023-04-07
10 days from now will be: 2023-04-17
30 days ago it was: 2023-03-08
In 2 months, it will be: 2023-06-07
100 days ago, it was: 2022-12-28

Deep Dive

Before Ruby absorbed date calculation functionalities into its standard and additional libraries, developers often had to manually calculate dates, considering leap years, different month lengths, and time zones—quite the headache.

The standard Date class does a lot out of the box. You can add (+) or subtract (-) days easily. However, for more intuitive time period manipulations, like “2 months from now”, we rely on active_support, extracted from Ruby on Rails. This gem uses extensions to standard Ruby classes, making such calculations human-friendly.

When calculating past or future dates, consider time zones if you’re also factoring in times (DateTime or Time objects). Ruby’s Time class and active_support can handle this but require a bit more setup.

Alternatives exist, like the time-lord and ice_cube gems, offering more syntactic sugar or specialized features (like recurring events), respectively.

See Also

Comparing two dates