Question

What is the smallest format I can have for the time element for example hh:mm or YYYY-MM-DD and still be valid for machines to read.

Answer 1

According to W3C, the smallest format is the time string:

A time consists of a specific time
with no time-zone information,
consisting of an hour, a minute, a
second, and a fraction of a second.

A string is a valid time string
representing an hour hour, a minute
minute, and a second second if it
consists of the following components
in the given order:

• Two digits, representing hour, in the
  range 0 ≤ hour ≤ 23
• A U+003A COLON
  character (:)
• Two digits, representing
  minute, in the range 0 ≤ minute ≤ 59
• Optionally (required if second is
  non-zero):
  • A U+003A COLON character
    (:)
  • Two digits, representing the
    integer part of second, in the range 0
    ≤ s ≤ 59
  • Optionally (required if
    second is not an integer):
    • A 002E FULL
      STOP character (.)
    • One or more digits,
      representing the fractional part of
      second

So: HH:MM is the smallest possible valid string.

Got that piece of documentation via the time element -> dateTime attribute -> dateTime attribute value -> valid date or time string -> valid time string. That’s a pretty comprehensive documentation…