So, you want to calculate some arbitrary date things? Easy! Let's use IRB! Open up the terminal (don't worry, there aren't any dragons here unless you've brought them). Then type irb and hit enter. it should bring up another prompt that looks something like this:
> irb
irb(main):001:0>
What this is is a REPL, which stands for Read-Evaluate-Print Loop. It reads what you type when you press enter, evaluates, and prints the result. If you want more information on REPLs, the railsbridge docs are a pretty good primer.
With our REPL open, type require "time", which loads the ruby Time class. Let's say we have a date we want to turn into a Time object to do stuff with. That would look like:
irb(main):001:0> require 'time'
=> true
irb(main):002:0> date = Time.parse("January 1, 3012")
=> 3012-01-01 00:00:00 -0800
Now we have a date object, and there are a lot of methods on date to try; you can see them by calling date.methods:
irb(main):010:0> date.methods
=> [:+, :-, :<=>, :eql?, :min, :inspect, :succ, :to_a, :to_s, :to_i, :to_f, :hash, :round, :ctime, :zone, :utc, :to_r, :localtime, :gmtime, :getlocal, :getgm, :getutc, :asctime, :sec, :hour, :mday, :day, :mon, :month, :year, :wday, :yday, :isdst, :dst?, :gmtoff, :gmt_offset, :utc_offset, :utc?, :gmt?, :sunday?, :monday?, :tuesday?, :wednesday?, :thursday?, :friday?, :saturday?, :tv_sec, :tv_usec, :usec, :tv_nsec, :nsec, :subsec, :strftime, :rfc2822, :rfc822, :httpdate, :xmlschema, :iso8601, :to_time, :to_date, :to_datetime, :<, :>, :<=, :>=, :==, :between?, :clamp, :remove_instance_variable, :instance_of?, :kind_of?, :is_a?, :tap, :public_send, :singleton_method, :instance_variable_defined?, :define_singleton_method, :method, :public_method, :instance_variable_set, :extend, :to_enum, :enum_for, :===, :=~, :!~, :respond_to?, :freeze, :object_id, :send, :display, :nil?, :class, :singleton_class, :clone, :dup, :itself, :taint, :tainted?, :untaint, :untrust, :untrusted?, :trust, :frozen?, :methods, :singleton_methods, :protected_methods, :private_methods, :public_methods, :instance_variable_get, :instance_variables, :!, :!=, :__send__, :equal?, :instance_eval, :instance_exec, :__id__]
The second one is what we need. - will let us subtract either two times from each other, or an integer number of seconds (You can also add two times, or add seconds to a time). So, to figure out our solution:
irb(main):008:0> date - 362_664*24*3600
=> 2019-01-22 00:00:00 -0800
We take the number of days we want to subtract, multiply that times 24 for hours, and times 3600 for seconds. Ruby will respect order of operations automatically when doing arithmetic.