func:toSpan

toSpan(x, tz: null)

Convert the following objects into a Span:

• Span: return itself
• Span+tz: update timezone using same dates only if aligned to midnight
• Str: return Span.fromStr using current timezone
• Str+tz: return Span.fromStr using given timezone
• DateTime..DateTime: range of two DateTimes
• Date..DateTime: start day for date until the end timestamp
• DateTime..Date: start timestamp to end of day for end date
• DateTime: span of a single timestamp
• DateSpan: anything accepted by toDateSpan() in current timezone
• DateSpan+tz: anything accepted by toDateSpan() using given timezone