[Ruby] Replace DateTime with Time

[wing328]

As per the conversation in #7648

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cc @cliffano (2017/07) @zlx (2017/09) @autopp (2019/02)

[jfeltesse-mdsol]

at a glance, you need to add require 'time' at the top of the ruby templates.

[wing328]

@jfeltesse-mdsol yup, thanks for the review. The CI also caught the issue.

Fixed via baf92ab