Fix #652 - Use internal/user datetime to calculate GMT offset

[scicali]

Use internal(from db field) / user(from form field) datetime to calculate GMT offset

Description

Pass internal/user datetime value to UserTimeZone function. It uses current datetime now, so if you insert/view a datetime in same timezone but different DST, calculated GMT offset is wrong

Motivation and Context

Fix #652

How To Test This

See #652

Types of changes

• [ x] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Final checklist

• My code follows the code style of this project found here.
• My change requires a change to the documentation.
• I have read the How to Contribute guidelines.