Bugfix JWT expiration convert

[yui10]

Fixed remaining bug in date conversion process in "JsonWebTokenEncoderGuiTool.cs" .

Pull request type

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• Bugfix
• New feature or enhancement
• UI change (please include screenshot!)
• Code style update (formatting, renaming)
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What is the current behavior?

Application crashes when entering decimals in date and time.

Issue Number: N/A

What is the new behavior?

• Changed from using Convert.ToInt32 to int.TryParse for date/time conversion.

Other information

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[veler]

Hello,
Thank you for your contribution, and sorry for the very late review. This PR looks good to me!