Ruby 对于任何给定日期（包括未来），我如何计算X天（如5天）之后的第二天（如周一）？

我一直在为这件事绞尽脑汁

下面是我目前的方法，只适用于过去的日期。如果开始日期是2个月后。。。它不起作用

def date_of_next_X_day_Y_days_out_from_Z_start(day=ENV['day_to_start_plans'], number_of_days=ENV['min_days_out'], start_date)
    # Explanation uses Monday as an example
    # When was the last Monday?
    date = Date.parse(day) 

    # Was the last Monday after the start_date? If so, then there's no difference between the last Monday and the next Monday; else, the next Monday is 7 days from the last Monday
    delta = date > (start_date) ? 0 : 7 
    actual_next_day = date + delta

    # Is there a minimum of Y number_of_days between the next Monday and the start_date? If not, bump to the following Monday by adding 7 more days
    delta_2 = (actual_next_day - start_date) < number_of_days ? 7 : 0
    actual_next_day + delta_2 #returned value
end

---

仅供参考，我确实有rails，所以请随时向advance或change helpers大声呼喊。

在纯Ruby中，我就是这样做的

def date_of_next_X_day_Y_days_out_from_Z_start(target_wday, skip, d)
  d += skip  # add "skip" number of days 

  # now keep add a day while checking wday (weekday) until we hit our target
  # 0 = Monday, ..., 6 = Sunday

  (0..6).each do |i|
    break if d.wday == target_wday
    d += 1
  end

  puts "Next Tuesday is #{d}"

用法示例：

# Find the next Tuesday (1) 5 days after June 15, 2015
date_of_next_X_day_Y_days_out_from_Z_start(1, 5, Date.parse("June 15,2015"))

---

将周天数用作星期天的数字0。。。6对于星期六，您可以使用以下公式确定所需日期：

def date_of_next_X_day_Y_days_out_from_Z_start(wday, number_of_days, start_date)
  day_ahead = start_date + number_of_days.days
  (day_ahead) + ((7 - day_ahead.wday + wday) % 7).days
end

一些测试：

dates = [
  { wday: 1, n_days: 5, start_date: Date.parse('May 23,2015') },
  { wday: 3, n_days: 7, start_date: Date.parse('May 8,2015') },
  { wday: 5, n_days: 3, start_date: Date.parse('June 15,2015') },
  { wday: 3, n_days: 1, start_date: Date.parse('May 13,2015') },
  { wday: 5, n_days: 1, start_date: Date.parse('May 20,2015') }
]

dates.each do |date|
  puts date_of_next_X_day_Y_days_out_from_Z_start(date[:wday], date[:n_days], date[:start_date])
end

# => 2015-06-01
# => 2015-05-20
# => 2015-06-19
# => 2015-05-20
# => 2015-05-22

这可能会起作用：

def date_of_next_X_day_Y_days_out_from_Z_start(day=ENV['day_to_start_plans'], number_of_days=ENV['min_days_out'], start_date)
  real_start_date = start_date + number_of_days     
  until real_start_date.wday == Date.parse(day).wday
    real_start_date += 1
  end  
  real_start_date
end

根据你目前的方法，问题是你没有考虑到你的开始日期是多少周前的日期，所以你的返回日期是短。您可以计算开始日期的前几周，并对delta_2重复计算，同时不断向其添加条件值。delta_2现在将包括1您的开始日期提前了多少周，2如果实际的下一天的天数少于要跳过的天数，则增加一周

def date_of_next_X_day_Y_days_out_from_Z_start(day=ENV['day_to_start_plans'], number_of_days=ENV['min_days_out'], start_date)
  real_start_date = start_date + number_of_days     
  until real_start_date.wday == Date.parse(day).wday
    real_start_date += 1
  end  
  real_start_date
end

def next_date(target_wday, skip_num_days, start_date)

  date = Date.parse(target_wday)
  delta = date > start_date ? 0 : 7
  actual_next_day = date + delta 

  # define delta_2 so you're able to use += 
  delta_2 = 0

  # calculates how many weeks ahead the start date is
  repeat = (actual_next_day - start_date).to_i % 7

  # removes the current week you're calculating
  ( repeat - 1 ).times do 
    delta_2 += (actual_next_day - start_date) < skip_num_days ? 7 : 0
  end

  actual_next_day + delta_2
end