How format Date in Ruby Google Calendar

Question

I am using the Ruby Quickstart Code from Google (https://developers.google.com/calendar/api/quickstart/ruby). I now want to format the date for a whole day. But I can't format the date:

Thats the code:

      {
        title: e.summary,
        start: e.start.date || e.start.date_time.strftime('%d.%m.%y %H:%M clock|%A')
      }
```
When I add strftime to "e.start.date", e.g. "e.start.date.strftime('%d.%m.%y %H:%M clock|%A')", I get the following error: 
undefined method `strftime' for nil:NilClass (NoMethodError).

How can I format the date? The date is currently in RFC3339