DateTime Ruby How to format correctly - ruby

Question

My project is supposed to fetch specific values from multiple hashes a put those values in a text file. Ideally what I need my code to do is to have every date for the employees be seven days apart, so the text file would look something like this:
"Rachel Thorndike
2017-10-09-T04:29:46-05:00
Stacie Smith
2017-10-16-T04:29:46-05:00"
What this is supposed to do is fetch employee's names and put the time of their "handoff" on the line under them. I looked online and found the DateTime that Ruby features but it looks like whatever I do isn't working. My code is this:
require 'date'
jsonUser["users"].each do |user|
somefile.puts user["user"]["summary"]
print 'Handoff Date + Time: '
parsed = DateTime.strptime(jsonUser["start"], '%d-%m-%Y %H:%M')
utc = parsed.next_day(7).strftime('%d-%m-%Y %H:%M')
puts utc
end
But terminal returns this code with an error 'strptime': invalid date (ArgumentError). Would anybody help me get this code to work the way I want to? Anything that points me to the right direction? With explanations, if it isn't too much.
Thank you so much!
Update
I was able to get the iso8601 to appear under their name. My new code is
require 'date'
jsonUser["users"].each do |user|
somefile.puts user["user"]["summary"]
print 'Handoff Date + Time: '
parsed = DateTime.iso8601(jsonUser["start"])
utc = parsed.next_day(7).iso8601
somefile.puts utc
end
BUT the .next_day method isn't increasing by 7 days that I want too. Thought? I have only got one value that is appearing and that is going under every line. Its the value of jsonUser["start"] + 7 days...so `2017-
10-16T04:29:46-05:00`
This is what parse.next_day(7) gives me.
"Sr Chid
Handoff Date + Time: 2017-10-16T04:29:46-05:00
Ash A
Handoff Date + Time: 2017-10-16T04:29:46-05:00
Ven D
Handoff Date + Time: 2017-10-16T04:29:46-05:00
Abhi S
Handoff Date + Time: 2017-10-16T04:29:46-05:00"
The value of jsonUser["start"] is 2017-10-09T04:29:46-05:00 so the good thing is that it did increase by 7 but it only did it once.
Update for Amadan
require 'date'
date = DateTime.iso8601(jsonUser["start"])
jsonUser["users"].each do |user|
if user["user"]["self"] == nil
nil
else
somefile.puts user["user"]["summary"].gsub(/\w+/, &:capitalize).gsub(/[.]/, ' ')
somefile.print 'Handoff Date + Time: '
date = date.next_day(7)
somefile.puts date.iso8061
end
end