Power Query: why is the timestamp in an earlier step later than in a later step?

Question

Power Query: why is the timestamp in an earlier step later than in a later step?

ONE_ME 0


let      one = Function.InvokeAfter(()=>DateTime.LocalNow(), #duration(0, 0, 0, 5)),      two = one  in      two

User's image

0 comments

1 answer

Your answer

Answer 1

DateTime.LocalNow() is a dynamic function. It can return a new value each time it is invoked during query evaluation.

In Power Query, steps are defined in a let ... in expression, and later steps reference earlier steps by variable name. In the sample:

let
    one = Function.InvokeAfter(()=>DateTime.LocalNow(), #duration(0, 0, 0, 5)),
    two = one
in
    two

one is the expression that waits 5 seconds and then evaluates DateTime.LocalNow(). two just references one.

The reason the timestamp shown for an earlier step can appear later than the timestamp shown for a later step is that DateTime.LocalNow() is not fixed. When the editor previews steps, Power Query can evaluate expressions at different times, so the value displayed for a step preview is not guaranteed to be the same moment across all step views.

If the same timestamp must remain constant throughout a single query evaluation, use a fixed current-time function instead, such as DateTime.FixedLocalNow(). Fixed variants return the same value throughout one query evaluation.