Add daytime() function

2 files changed, 38 insertions, 0 deletions

diff --git a/libbutl/timestamp.cxx b/libbutl/timestamp.cxx
index 296e070..0242758 100644
--- a/libbutl/timestamp.cxx
+++ b/libbutl/timestamp.cxx
@@ -662,4 +662,32 @@ namespace butl
     return system_clock::from_time_t (time) +
       chrono::duration_cast<duration> (t.second);
   }
+
+  duration
+  daytime (timestamp t)
+  {
+    // Convert the time from Epoch to the local time.
+    //
+    time_t time (system_clock::to_time_t (t));
+
+    std::tm tm;
+    if (details::localtime (&time, &tm) == nullptr)
+      throw_generic_error (errno);
+
+    // Roll the time back to the latest midnight.
+    //
+    tm.tm_hour = 0;
+    tm.tm_min = 0;
+    tm.tm_sec = 0;
+
+    // Convert the local midnight to time from Epoch.
+    //
+    time = mktime (&tm);
+    if (time == -1)
+      throw_generic_error (errno);
+
+    // Note: preserves the accuracy of the original time.
+    //
+    return t - system_clock::from_time_t (time);
+  }
 }
diff --git a/libbutl/timestamp.mxx b/libbutl/timestamp.mxx
index f4a06f9..9525ec0 100644
--- a/libbutl/timestamp.mxx
+++ b/libbutl/timestamp.mxx
@@ -194,4 +194,14 @@ LIBBUTL_MODEXPORT namespace butl
                const char* format,
                bool local,
                const char** end = nullptr);
+
+  // Rebase a time point from UNIX epoch to midnight in the local time zone
+  // (so the returned duration is always less than 24 hours).
+  //
+  // Specifically, convert the time point from Epoch to the local time and
+  // return the time elapsed since midnight. Throw std::system_error on
+  // underlying time conversion function failures.
+  //
+  LIBBUTL_SYMEXPORT duration
+  daytime (timestamp);
 }