Daily DAX : Day 456 ISDATETIME

DAX Function ISDATETIME


 In Power BI, ISDATETIME is an Information function in DAX used to validate whether a specific value or expression results in a DateTime data type. It is essentially a "safety check" for your data logic, ensuring that the input you are passing into other functions is actually a date or time before you try to calculate something with it.


1. Syntax and Return Value

The syntax is straightforward:


ISDATETIME(<value>)

    • Value: Any DAX expression or column reference you want to check.

    • Returns: A Boolean value—TRUE if the value is a date/time; FALSE otherwise.


2. Key Use Cases

While Power BI usually handles data types automatically, ISDATETIME becomes critical in scenarios where data is "dirty" or polymorphic (mixed types).

A. Data Validation in Conditional Logic

If you have a column that might contain mixed data (like a "Notes" column where some entries are dates and others are text), you can use ISDATETIME to prevent your measures from breaking.

Example:

If the value is a date, calculate the days since today; otherwise, return a "Check Data" message.

Code snippet

Date Check = 

IF(

    ISDATETIME(SourceTable[RawValue]), 

    DATEDIFF(SourceTable[RawValue], TODAY(), DAY), 

    BLANK()

)

B. Error Prevention in Complex Calculations

When nesting multiple functions, a non-date value can cause an entire visual to crash with an "Mismatched types" error. ISDATETIME acts as a guardrail.

C. Distinguishing Between Numbers and Dates

In DAX, dates are technically stored as decimal numbers (e.g., 45292.0 is January 1, 2024). However, ISDATETIME specifically checks the metadata/data type of the column or expression, not just whether the number could be a date.


3. Important Remarks

    • DirectQuery Limitation: This function is not supported for use in DirectQuery mode when used in calculated columns or Row-Level Security (RLS) rules.

    • Literal Support: You can check DAX literals using the dt prefix, such as ISDATETIME(dt"2024-01-01"), which would return TRUE.

    • Standard Comparisons: | Input | Result | | :--- | :--- | | ISDATETIME("2024-01-01") | FALSE (This is a String) | | ISDATETIME(DATE(2024,1,1)) | TRUE | | ISDATETIME(45292) | FALSE (This is a Number) |


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