Ruby parse DateTime with variable - ruby-on-rails

Question

I'm trying to parse a specific hour of a specific date. When I put the date directly as an argument, it works fine, but when I create a variable and put it in the argument it returns the current date.
Why is that?
NOTE: the variable time is 9pm and I need to parse 9pm of 12 March 2016.
datetime = DateTime.new(2016,3,12,9)
=> Sat, 12 Mar 2016 09:00:00 +0000
DateTime.parse("sat 12 march 2016 9pm")
=> Sat, 12 Mar 2016 21:00:00 +0000
DateTime.parse("datetime 9pm")
=> Mon, 14 Mar 2016 21:00:00 +0000

Answer 1

In your third call, you use the literal string "datetime" rather than the value of your datetime variable. You can use string interpolation to use the variable's value:
DateTime.parse("#{datetime} 9pm")
In this case, the "9pm" is ignored since it doesn't make sense added to the end of an existing date but this is why the initial attempt wasn't working. Interpolation is generally a solution for using a variable's value rather than its name.
If your goal is to change the time of an existing date, use the change method:
datetime.change(hour:21)

Answer 2

You can also try this
date = Date.new(2016,3,12)
DateTime.parse("#{date} 9pm")
## Output
Sat, 12 Mar 2016 21:00:00 +0000
OR
datetime = DateTime.new(2016,3,12,9)
DateTime.parse((datetime + 12.hours).to_s)
## Output
Sat, 12 Mar 2016 21:00:00 +0000
OR
DateTime.parse((datetime + 12.hours).to_s).strftime("%a, %d %b %Y %I:%M %p")
## Output
Sat, 12 Mar 2016 09:00 PM