daysdiff() #@ Find the number of days between two dates
{          #@ USAGE: daysdiff DATE1 DATE2 [VAR]
  local _d1 _d2 fmt var=$3 _df
  datenum "$1" _d1
  datenum "$2" _d2
  _df=$(( _d2 - _d1 ))
  is_var "$var" && printf -v "$var" %d  "$_df" || vshow _df
}