meetup exercise: off-by-one error

[glennj]

For Date.UTC, the month value is a month index. I was not decrementing by one.

Also, changed Date.UTC to new Date to avoid any timezone related issues.

attn: @BNAndras

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

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[no important files changed]

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[glennj]

This is a change to the spec, so existing solutions should be re-tested: "yes important files changed"