問：

給定一個 DataTime 值，如何計算如下時間？比如說：

• 2 小時前？
• 3 天前？
• 1 個月前？

答：

我在 DateTime 類上做了一個擴展方法，你可以給它傳遞未來或者過去的時間，還可以給他傳一個 approximation 選項來指定更精細的信息描述，參考如下代碼：

using System.Text;/// <summary> /// Compares a supplied date to the current date and generates a friendly English /// comparison ("5 days ago", "5 days from now") /// </summary> /// <param name="date">The date to convert</param> /// <param name="approximate">When off, calculate timespan down to the second. /// When on, approximate to the largest round unit of time.</param> /// <returns></returns> public static string ToRelativeDateString(this DateTime value, bool approximate) {StringBuilder sb = new StringBuilder();string suffix = (value > DateTime.Now) ? " from now" : " ago";TimeSpan timeSpan = new TimeSpan(Math.Abs(DateTime.Now.Subtract(value).Ticks));if (timeSpan.Days > 0){sb.AppendFormat("{0} {1}", timeSpan.Days,(timeSpan.Days > 1) ? "days" : "day");if (approximate) return sb.ToString() + suffix;}if (timeSpan.Hours > 0){sb.AppendFormat("{0}{1} {2}", (sb.Length > 0) ? ", " : string.Empty,timeSpan.Hours, (timeSpan.Hours > 1) ? "hours" : "hour");if (approximate) return sb.ToString() + suffix;}if (timeSpan.Minutes > 0){sb.AppendFormat("{0}{1} {2}", (sb.Length > 0) ? ", " : string.Empty, timeSpan.Minutes, (timeSpan.Minutes > 1) ? "minutes" : "minute");if (approximate) return sb.ToString() + suffix;}if (timeSpan.Seconds > 0){sb.AppendFormat("{0}{1} {2}", (sb.Length > 0) ? ", " : string.Empty, timeSpan.Seconds, (timeSpan.Seconds > 1) ? "seconds" : "second");if (approximate) return sb.ToString() + suffix;}if (sb.Length == 0) return "right now";sb.Append(suffix);return sb.ToString(); }

github 上有一個非常流行的 DateTime 幫助類，可以非常精細化的滿足你的要求，參見網址：https://github.com/FluentDateTime/FluentDateTime ， 比如下面這些例子：

var dateTime1 = 2.Hours().Ago(); var dateTime2 = 3.Days().Ago(); var dateTime3 = 1.Months().Ago(); var dateTime4 = 5.Hours().FromNow(); var dateTime5 = 2.Weeks().FromNow(); var dateTime6 = 40.Seconds().FromNow();

純手工封裝，用 SortedList 預先做一個映射，應該還是能夠滿足你的需求，參考如下代碼。

static readonly SortedList<double, Func<TimeSpan, string>> offsets = new SortedList<double, Func<TimeSpan, string>> {{ 0.75, _ => "less than a minute"},{ 1.5, _ => "about a minute"},{ 45, x => $"{x.TotalMinutes:F0} minutes"},{ 90, x => "about an hour"},{ 1440, x => $"about {x.TotalHours:F0} hours"},{ 2880, x => "a day"},{ 43200, x => $"{x.TotalDays:F0} days"},{ 86400, x => "about a month"},{ 525600, x => $"{x.TotalDays / 30:F0} months"},{ 1051200, x => "about a year"},{ double.MaxValue, x => $"{x.TotalDays / 365:F0} years"} };public static string ToRelativeDate(this DateTime input) {TimeSpan x = DateTime.Now - input;string Suffix = x.TotalMinutes > 0 ? " ago" : " from now";x = new TimeSpan(Math.Abs(x.Ticks));return offsets.First(n => x.TotalMinutes < n.Key).Value(x) + Suffix; }

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