Added ToClientTimezone() util func.

pkg/util/util.go

@@ -0,0 +1,21 @@
+package util
+
+import (
+	"net/http"
+	"time"
+)
+
+// ToClientTimezone converts given time to timezone set in a
+// X-Client-Timezone header. If this header is not set, then
+// converts to UTC.
+func ToClientTimezone(t time.Time, r *http.Request) time.Time {
+	if tz := r.Header.Get("X-Client-Timezone"); tz != "" {
+		loc, err := time.LoadLocation(tz)
+		if err != nil {
+			return t.UTC()
+		}
+		return t.In(loc)
+	}
+
+	return t.UTC()
+}