我正在创建一个数据库，用于计算两个里程碑之间经过的时间长度。
[ID] [Milestone] [MDate] [Length of Time]
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1 1 Jan 1, 2013 [If the following milestone is 2 and they have both the same ID;
Calculate Datediff of Jan 1, 2013 vs Feb 1, 2013] Ans: 1 month]
1 2 Feb 1, 2013 [if no milestone that followed, end date should be date(); Datediff= Feb 1, 2013 vs. Date()]
2 1 Jan 15, 2013 [If the following milestone is 2 and they have both the same ID;
Calculate Datediff of Jan 15, 2013 vs Feb 1, 2013] Ans: 18 days]
2 2 Feb 1, 2013 [If the following milestone is 3 and they have both the same ID;
Calculate Datediff of Feb 1, 2013 vs Feb 27, 2013] Ans: 12 days]
2 3 Feb 27, 2013 if no milestone that followed, end date should be date(); Datediff= Feb 27, 2013 vs. Date()]
表的设置是：
[ID] - Number;
[Milestone] - Number;
[Mdate]-Date;
[Length of time] - calculated field in the query; should be with a user defined function
问题是我不能自动计算两者之间的时间长度。我需要我的数据库来感知当具有相同ID和里程碑编号的两个记录被更改时，前一个里程碑将使用以下里程碑的结束日期，否则如果结束日期之后没有任何内容是当前日期。
我需要一个自动计算时间长度的用户自定义函数。