980604-1 Specify, using ABNF, a syntax for the date format shown by the
example 1998-06-04T12:34:59+01:00, and where the minutes and
seconds are allowed to be omitted, and where the character between
date and time is either "T" or "t".

Suggested solution 1: 

date = 4D "-" 2D "-" 2D ("T"/"t") 2D [":" 2D [":" 2D]]
       ("+"/"-") 2D [":" 2D]

D    = %d48-&d57

Suggested solution 2: 

date = year "-" month "-" day ("T"/"t") time-of-day
                   ("+"/"-") time-zone-offset
year             = 4D
month            = ("0" / "1") D 
day              = ("0" / "1" / "2" / "3") D
time-of-day      = hour [":" sixtile] [":" sixtile]
time-zone-offset = hour [":"sixtile]
hour             = ("0" / "1") D
sixtile          = ("0" / "1" /"2" / "3" / "4" / "5") D
D                = "0" / "1" / "2" / "3" / "4" / "5" /
                   "6" / "7" / "8" / "9"

Suggested solution 3: 

date = DIGIT DIGIT DIGIT DIGIT "-" DIGIT DIGIT 
       "-" DIGIT DIGIT ("T"/"t") DIGIT DIGIT
       [":" DIGIT DIGIT [":" DIGIT DIGIT]]

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