EXIF alone usually won’t give sub-second capture timing. A practical way to calibrate your camera is to photograph a precise clock synchronized to UTC, then compare the visible clock time to the EXIF time. If the clock only shows whole seconds, shoot a burst and see between which frames the displayed second changes; that can improve timing to roughly a fraction of a second depending on burst rate.

For best results, determine whether your camera records the exposure time at shutter open or shutter close, since that affects how you interpret the timestamp and exposure duration. Including a precise clock in the frame is the most direct way to measure the timing offset and any camera clock drift.

Shutter lag does exist between button press and actual exposure, and it can matter for your application. The exact amount depends on the camera and shooting conditions, so you should measure it experimentally for your setup rather than rely on EXIF metadata. Also remember that any visible or logical time reference is effectively averaged or blurred over the exposure duration, so you must decide whether you need the start, end, or midpoint of the exposure.