Mark times on the day before a work day as -1

A time on the day after its work day shows "+1" (a night shift ending at
04:10 +1). A time on the day before, which a start backdated across
midnight can produce, showed "+-1". It now shows "-1", on the Days
screen and in Markdown reports.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/export/markdown.rs

@@ -80,12 +80,13 @@ pub fn render(report: &Report) -> String {
     lines.join("\n") + "\n"
 }
 
-/// The end time, marked "(+1)" when it falls on the next calendar day.
+/// The end time, marked "(+1)" when it falls on the next calendar day (or
+/// "(-1)" the day before).
 fn out_time(report: &Report, day: &WorkDay, end: Timestamp) -> String {
     let days_later = report.local(end).date().duration_since(day.date()).as_hours() / 24;
     match days_later {
         0 => report.time(end),
-        later => format!("{} (+{later})", report.time(end)),
+        later => format!("{} ({later:+})", report.time(end)),
     }
 }
 

src/ui/component.rs

@@ -92,12 +92,14 @@ impl Context<'_> {
         self.fmt.time(self.local(moment).time())
     }
 
-    /// A clock time, marked "+1" when it falls the day after `day`.
+    /// A clock time, marked "+1" when it falls the day after `day` (or "-1"
+    /// the day before, which a backdated start can do).
     pub fn time_on(&self, moment: Timestamp, day: Date) -> String {
         let days_later = self.local(moment).date().duration_since(day).as_hours() / 24;
         match days_later {
             0 => self.time(moment),
-            later => format!("{} +{later}", self.time(moment)),
+            // `:+` always writes the sign: +1, -1.
+            later => format!("{} {later:+}", self.time(moment)),
         }
     }