Date the composite at the mean of its frames' capture times

It carried the first frame's, so it sorted before the sweep it was made
from. The middle of the sweep puts it among them.

ui/dr-ui/src/merge.rs

@@ -518,7 +518,16 @@ fn run_inner(
     let black = first.raw.black_level[0];
     let white_level = u32::from(first.raw.white_level.saturating_sub(black)).max(1);
     let profile = dng_profile(first, white_level);
-    let (_, carried) = crate::export::header_for_file(&first.meta);
+    let (_, mut carried) = crate::export::header_for_file(&first.meta);
+    // The composite is dated at the middle of its sweep — the mean of the
+    // frames' capture times — so it sorts among the frames it was made
+    // from, not at the first of them and not at the moment of the merge.
+    // The zone is the first frame's; a sweep does not cross one.
+    let stamps: Vec<i64> = frames.iter().filter_map(|f| f.meta.captured_at).collect();
+    if !stamps.is_empty() {
+        let sum: i128 = stamps.iter().map(|&t| t as i128).sum();
+        carried.captured_at = Some((sum / stamps.len() as i128) as i64);
+    }
 
     let output = MergeOutput {
         projection,