Required delimiter is missing

The fully-formatted error message

This error message complains that the input doesn't specify the timezone offset. In reality, this is not necessarily illegal, such templates simply mean a (kind of careless) local time. But, presume that in this example, the timezone offset information is necessary.

The Timezone offset expression is missing from the end of the input:

          1992-05-29?12:25:12.1
                              ^X

Consider supplying 'Z', a minus timezone offset (example '-04:00'), or positive timezone offset (example '+07:00').

Substitutions

Name	Value	Type	Source
Input	1992-05-29?12:25:12.1	String	The ISO8601 string being parsed
Expected	'Z', '+', or '-'	Char	What the parse command is looking for
Actual	Nothing	Maybe	What the parse found
PreviousName	"fraction"	String	The name of the field immediately preceding the erroneous delimiter
PreviousValue	1	Int	The value of the immediately preceding field
PreviousType	digits	String	"digits"
PreviousStartOffset	19	Int	The displacement from the start of the string being parsed to locate the leftmost '^'
PreviousCharCount	1	Int	The number of '^' chars to position under the previous value
TopLine	"The ? is missing from ?:"	String	This error category
BottomLine	"A Z, +offset, or -offset belongs in that position."	String	This error category

Notes

1. The Value column above describes the example values in the sample error message above.
2. The offset of the actual error position is calculated: it is always after the sum of the PreviousStartOffset and the PreviousCharCount.
3. Here the number of 'X' is one. But Digit sequence has wrong length has no 'X' because the error field is the first one.
4. Note that TopLine and BottomLine have '?' marking where values will be substituted.
5. I'm wondering if the type of the preceding field can be automatically derived.