A conservative extension of ISO 8601 to support fractional days is a fun blog post about extending the ISO format for timestamps to support fractional days in addition to fractional seconds, minutes, and hours.
Aesthetically I dislike the "T" in ISO 8601 format, but what can you do.
Fractional days allows for easier handling of the French Republican Calendar and Swatch Internet Time!
ISO 8601 does not allow for specifying offsets from UTC in seconds (another obvious oversight).
in contrast to common scientific usage, the decimal part is recommended to be separated by a comma and not a full stop, although the latter is permitted too.
I had no idea!
Anyway, what's next - fractional months and fractional years? Why not?