You aren't required to provide the offset (IE -07:00). If you don't provide the offset it would use the user's time zone (or system time zone if the user doesn't have one configured). For the REST API this would be the user you're authenticating as/created an API key for but for a standard integration this would typically be MXINTADM. As long as you don't need to support multiple time zones, I would just avoid sending the offset.
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Steven Shull
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