Timing of fluorescence measurements relative to ephys for GCaMP data

I'm interested in comparing ephys. and fluorescence in the GCaMP6 datasets.

My question is, how are the fluorescence measurement times defined? Is it the frame signal at the start of each image frame, or is the time the center of the cell is scanned (taking into account the cell's position within the frame)?

If I look in the processed data files, I can get spike times as follows:

st = obj.timeSeriesArrayHash.value{5}.time(obj.timeSeriesArrayHash.value{5}.valueMatrix);

and fluorescence measurement times as follows:

ft = obj.timeSeriesArrayHash.value{1}.time;

So I want to know what the precise definition of ft is in terms of the fluorescence data acquisition process.

Many thanks!