XBRL-based structured digital financial reporting - Blog: Digital Financial Reporting (using XBRL)

Most SEC XBRL financial filings articulate the current balance sheet date and income statement start date correctly. Some don't though. Most of the time the issue is just sloppiness and the lack of a business rule enforced by software to keep these dates correct and consistent.

However, for 10-Qs for the second and third quarters, there is a higher potential for ambiguity.

You can refer to this PDF to understand what I am taking about below.

The current balance sheet date is easy. All of the following should be consistent:

the fact value reported for the concept "dei:DocumentPeriodEndDate"
the endDate of the period for the context of the concept dei:DocumentPeriodEndDate (which is also the required context per the EFM)
the instant of the period of the actual facts reported on the balance sheet (i.e. you don't want to be saying your balance sheet is one date and actually use some other date, some filers make this mistake)

So, all those should be consistent and most SEC filers get this correct.

The income statement is easy also for a 10-K or a 10-Q for the first quarter. These are the pieces which must be consistent:

the startDate of the period of the context of the concept dei:DocumentPeriodEndDate (which is also the required context per the EFM)
the endDate of the income statement is the balance sheet date, so the current income statement end date is the same as the current balance sheet (so follow the rules above)
the duration period (startDate and endDate) of the actual facts reported on the income statement (i.e. don't be inconsistent)

HOWEVER, if the filer is reporting a Q2 or Q3 10-Q and there is year-to-date and quarter-to-date information, what should you use as the startDate of the required context? YTD or QTD start date?

Well, the logical answer is to be consistent with the 10-K and 10-Q for Q1 which is to use the YTD startDate as the value of the required context. The EFM is unclear. Most SEC filers use YTD, but some use the QTD.