From: "Mike Abbott" <AE-List@media-motion.tv>
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Subject: Re: [AE] Footage duration expression
To: After Effects Mail List <AE-List@media-motion.tv>
References: <list-7508030@media-motion.tv>
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Date: Mon, 17 Feb 2020 21:31:15 +0000
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On 17/02/2020 20:19, Phil Spitler wrote:
> According the the expressions menu I should be able to get the 
> duration of a layer but my expressions keep erroring…
> Saying the property or class “duration” is missing.
> My expression is:
> movieLength=thisComp.layer(“source").duration;


Try:
movieLength = thisComp.layer(“source").source.duration;

...but that will give you the source duration - not the trimmed 
duration, and wouldn't work for stills I don't expect.

Would this work?

myMovie = thisComp.layer("source");
movieLength = myMovie.outPoint - myMovie.inPoint


Mike A.