Good idea! I wish it was this simple, but if you make DateTime parametric it becomes a non-concrete type (when spelled without the type parameters). So user code which expects it to be a concrete type (backed by the bits of an Int64) will suddenly become really slow. For example, consider code which constructs a Vector{DateTime}() and then fills it with a loop. Or any struct containing a DateTime, which suddenly gets a pointer in it rather than an inlined Int64.
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