You count from the day you received the confirmation email from Sheffield... because that is the day Sheffield log your application into the system and start their processing time count.
In other countries you submit your documents at the biometrics appointment, so it makes sense to count from that date in those countries... but in the US you mail your documents yourself after attending biometrics.
Since Sheffield have no control over how long it takes you to mail the documents, it wouldn’t make sense for them to include that time in their processing time calculation.
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