==== if it is a digit
- the string may contain: NNN d NNN h NNN m NNN s
- This represents NNN days, NNN hours, NNN minutes and NNN seconds.
+ the string may contain: NNN Y NNN M NNN W NNN d NNN h NNN m NNN s
+ This represents NNN years, NNN months, NNN weeks, NNN days, NNN hours,
+ NNN minutes and NNN seconds.
The embeded white space is optional.
These terms must appear in this order.
+ Case is significant: 'M' is months and 'm' is minutes.
The final "s" is optional.
All of the terms ("NNN" plus designator) are optional.
Minutes and seconds may optionally be represented as NNN:NNN.