Round timestamps to whole minutes on creation

The displayed timestamps were always floored, as were calculated
difference and its sum, which meant a duration that seemed like 1 hour
was more often than not displayed as 0:59 instead of 1:00.

src/timesheet.rs

@@ -35,7 +35,8 @@ impl TimePoint {
 }
 
 fn now() -> Time {
-    OffsetDateTime::now_local().time()
+    let raw_time = OffsetDateTime::now_local().time();
+    Time::try_from_hms(raw_time.hour(), raw_time.minute(), 0).unwrap()
 }
 
 impl fmt::Display for TimePoint {