Workaround to use get_date_from_gmt to format into 'U'

[mircobabini]

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Changes proposed in this Pull Request:

Fix the next payment date TIME (and the cancel date TIME if using CONPD in core) to consider the timezone.
When usign get_date_from_gmt with format = 'U', the result does not consider the timezone.
Then, a solution is to call that function twice. One to add the timezone, then to format the datetime.

How to test the changes in this Pull Request:

1. Create a new subscription
2. Check the TIME of the next payment/cancellation

It will be a cycle ahead (MINUS or PLUS the timezone).

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

[dparker1005]

Overall, looks good to me.

I hesitated a bit when initially reviewing this PR since 'U' is, by definition, the seconds since the Unix Epoch (January 1 1970 00:00:00 GMT). Altering the date to a "local timestamp" like this PR breaks that definition.

With that being said, creating a "local timestamp" like this is something that we already do elsewhere in PMPro, so this change would bring the PMPro Subscriptions class more in line with other PMPro code.

I think we should merge this change and, down the line, consider setting stronger standards about how we handle timestamps in PMPro.