Asking how long somebody waited to be greeted, or how long from placing order to receiving order, etc. are very common questions and there is a matching question type in Sassie already for those that choose to do imports.
Asking two exact times of day and then doing a calculation to get time length is a very cumbersome way to do this and is asking the shopper to remember 2x as many numeric values. It’s far easier for a shopper to remember a wait time of 1m43s than to remember I entered the line at 12:43:20 and got my food at 12:45:03. Especially if multiple time lengths are being measured during a visit.
And asking shoppers to simply enter total of number of seconds, ie. asking them to do the math, is a recipe for human error.