Go calendar Date (only), clock Time (only), and nullable versions of same, with sensible JSON and SQL behavior.

I wrote this because I needed:

• calendar dates (with no associated time)
• clock times (with no associated date)
• with no time zone (time zone to be stored separately as an Olsen TZDB value)
• with support for nullable dates/times
• compatible with PostgreSQL DATE and TIME types, and
• with sensible ISO-8601 bare-date and bare-time JSON representations

So I can define a structure with (say):

StartDate         calendar.Date     `json:"start_date"`
StartTime         calendar.NullTime `json:"start_time"`
EndDate           calendar.Date     `json:"end_date"`
EndTime           calendar.NullTime `json:"end_time"`
TimeZone          string            `json:"timezone"`	

and scan in values from a PostgreSQL database, and serialize them to JSON, and get out something like:

…"start_date":"2019-02-14","start_time":"09:00:00","end_date":"2019-02-14","end_time":"17:00:00","timezone":"Americas/Chicago"…

If you don't have those precise requirements, this is not the date/time library for you.