anchor.diff:  
   ** For seconds, the maximum number is equivalent to approximately 68 years. 
   
  -** For weeks, if a difference in weeks is requested, one of the two dates cannot precede 0001-01-07.
  +** For weeks, the dates must be later than 0001-01-07.
   
   <<<
   [[examples_of_datediff]]
  
positive.code: [[the_first_week_of_year]]
  ==== The first week of year
  
  This value cannot be specified, the default value is the week in which Jan 1 occurs.  
  
  [[negative_or_zero]]
  ==== Negative or zero
  
negative.code: However, the DATEDIFF difference is 1 minute, 1 hour, 1 day, 0 week^1^, 1 month, 1 quarter or 1 month depending on the specified _datepart_.
  
  +
  ^1^ NOTE: 2017-12-31 (_startdate_) falls on a Sunday, which is in the same week as 2018-01-01. For more information, see <<the_first_day_of_week,The first day of week>>.
  
  * Likewise, the difference between 2017-01-01 and 2018-12-31 is almost two years.
  
  