Gaps in date-time format string support

The supported format string tokens fail to cover:

• one-or-two digit number using space (not zero) for padding when single-digit
• include sign even if + (probably only relevant for year; the ±HH:mm offset is now covered by tt or ttt)
• distinction between standalone and inline forms of names for days and months

We should check for any other formatting quirks that might reasonably be hoped for and work out how to support them, where relevant.