#
#
# patch "dates.cc"
#  from [f2cbec98589e45cd6f5cf8a1e7d69e6f54f8defd]
#    to [9bcad49bcaea432aa27a368de34027c582c0c960]
# 
# patch "dates.hh"
#  from [96d6837b825444479af1813301dcd3216881167e]
#    to [bea2c2fefa4a7464ec575bdcc2ae8f4ad40961b8]
#
============================================================
--- dates.cc	f2cbec98589e45cd6f5cf8a1e7d69e6f54f8defd
+++ dates.cc	9bcad49bcaea432aa27a368de34027c582c0c960
@@ -437,30 +437,47 @@ date_t &
 }
 
 date_t &
-date_t::operator +=(u64 const & other)
+date_t::operator +=(s64 const other)
 {
-  // prevent overflows
-  I(other < u64_C(253402300800000));
+  // only operate on vaild dates (i..e. d != 0)
+  I(valid());
 
   d += other;
 
   // make sure we are still before year 10'000
   I(d < u64_C(253402300800000));
 
+  // make sure the date is still valid
+  I(valid());
+
   return *this;
 }
 
 date_t &
-date_t::operator -=(u64 const & other)
+date_t::operator -=(s64 const other)
 {
-  // prevent underflows
-  I(d >= other);
-  d -= other;
-  return *this;
+  // simply use the addition operator with inversed sign
+  return operator+=(-other);
 }
 
+date_t
+date_t::operator +(s64 const other) const
+{
+  date_t result(d);
+  result += other;
+  return result;
+}
+
+date_t
+date_t::operator -(s64 const other) const
+{
+  date_t result(d);
+  result += -other;
+  return result;
+}
+
 s64
-date_t::operator -(date_t const & other)
+date_t::operator -(date_t const & other) const
 {
   return d - other.d;
 }
@@ -740,9 +757,9 @@ UNIT_TEST(date, comparisons)
   UNIT_TEST_CHECK(v == jun);
 
   // check limits for subtractions
-  v = date_t(12345000);
+  v = date_t(12345000 + 1000);
   v -= 12345000;
-  UNIT_TEST_CHECK(v == date_t::from_string("1970-01-01T00:00:00"));
+  UNIT_TEST_CHECK(v == date_t::from_string("1970-01-01T00:00:01"));
   UNIT_TEST_CHECK_THROW(v -= 1000, std::logic_error);
 
   // check limits for additions
============================================================
--- dates.hh	96d6837b825444479af1813301dcd3216881167e
+++ dates.hh	bea2c2fefa4a7464ec575bdcc2ae8f4ad40961b8
@@ -57,11 +57,13 @@ struct date_t
     { return d != other.d; };
 
   // Addition and subtraction of millisecond amounts
-  date_t & operator +=(u64 const & addend);
-  date_t & operator -=(u64 const & addend);
+  date_t & operator +=(s64 const other);
+  date_t & operator -=(s64 const other);
+  date_t operator +(s64 const other) const;
+  date_t operator -(s64 const other) const;
 
   // Difference between two dates in milliseconds
-  s64 operator -(date_t const & other);
+  s64 operator -(date_t const & other) const;
 
 private:
   // The date as an unsigned 64-bit count of milliseconds since