SSRS日期表达式问题：我的首个DateDiff写法为何无法正常运行？

Hey there! Let's figure out why your first SSRS DateDiff expression isn't working—since you got a second version to run, it's almost definitely one of these common missteps with how SSRS handles this function:

• 参数顺序搞反了
  SSRS’s DateDiff follows the syntax: DateDiff(interval, startDate, endDate). A super common mistake is swapping the start and end dates. Not only will this give you negative values (which might not be what you expect), but if either date is in an incompatible format or has nulls, it can cause the whole expression to fail. If your first expression had the dates reversed compared to the working one, that’s probably the issue.

• 间隔参数格式不正确
  SSRS requires the interval parameter to be a specific quoted string, like "yyyy" for years, "d" for days, "h" for hours, etc. If your first expression used an invalid value (like year instead of "yyyy") or forgot the quotes entirely, SSRS won’t recognize the interval and will throw an error.

• 未处理日期数据类型不匹配
  If your date fields are stored as strings instead of proper DateTime types, feeding them directly into DateDiff will cause conversion errors. Maybe your working expression added a CDate() wrapper (like CDate(Fields!OrderDate.Value)) to convert the string to a date, while your first one didn’t—this would make the first expression fail outright.

• 忽略了NULL值
  SSRS doesn’t handle null dates gracefully in DateDiff; if either the start or end date is null, the function will return null or throw an error. If your working expression added null handling (like IIF(IsNull(Fields!StartDate.Value), Today(), Fields!StartDate.Value)), but your first one didn’t, that’s another likely culprit.

If you can share both your failing expression and the working one, I can give you a precise, line-by-line breakdown. But based on the most common SSRS DateDiff issues, these are the top candidates for what went wrong.

内容的提问来源于stack exchange，提问作者Jagjit Saini