RFC 4517 says: The "Z" form of <g-time-zone> SHOULD be used in preference to <g-differential>.
The primary reason for this is interoperability. Not all implementations can deal (well) with g-differential. It
should be avoided (on the wire).
Another consideration is that g-differential may be different in a given location, depending on the date.
For example, in California, we are -0800 during the fall/winter and -0700 during the spring/summer due to daylight savings silliness.
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