diff --git a/chrono/datetime.cpp b/chrono/datetime.cpp
index 863098d..51cd68b 100644
--- a/chrono/datetime.cpp
+++ b/chrono/datetime.cpp
@@ -50,7 +50,6 @@ template <typename num1, typename num2, typename num3> constexpr bool inRangeExc
  * - Add method for printing to custom string formats.
  * - Allow to determine the date part for each compontent at once to prevent multiple
  *   invocations of getDatePart().
- * - Make more methods constexpr.
  */
 
 /*!
@@ -59,7 +58,7 @@ template <typename num1, typename num2, typename num3> constexpr bool inRangeExc
 DateTime DateTime::fromTimeStamp(time_t timeStamp)
 {
     if (timeStamp) {
-        struct tm *timeinfo = localtime(&timeStamp);
+        struct tm *const timeinfo = localtime(&timeStamp);
         return DateTime::fromDateAndTime(timeinfo->tm_year + 1900, timeinfo->tm_mon + 1, timeinfo->tm_mday, timeinfo->tm_hour, timeinfo->tm_min,
             timeinfo->tm_sec < 60 ? timeinfo->tm_sec : 59, 0);
     } else {
@@ -67,14 +66,6 @@ DateTime DateTime::fromTimeStamp(time_t timeStamp)
     }
 }
 
-/*!
- * \brief Constructs a new DateTime object with the GMT time from the specified UNIX \a timeStamp.
- */
-DateTime DateTime::fromTimeStampGmt(time_t timeStamp)
-{
-    return DateTime(DateTime::unixEpochStart().totalTicks() + static_cast<std::uint64_t>(timeStamp) * TimeSpan::ticksPerSecond);
-}
-
 /*!
  * \brief Parses the given C-style string as DateTime.
  * \throws Throws a ConversionException if the specified \a str does not match the expected time format.
@@ -189,18 +180,6 @@ std::pair<DateTime, TimeSpan> DateTime::fromIsoString(const char *str)
     return make_pair(DateTime::fromDateAndTime(values[0], values[1], *dayIndex, *hourIndex, values[4], *secondsIndex, miliSeconds), delta);
 }
 
-/*!
- * \brief Returns the string representation of the current instance using the specified \a format.
- * \remarks If \a noMilliseconds is true the date will be rounded to full seconds.
- * \sa toIsoString() for ISO format
- */
-string DateTime::toString(DateTimeOutputFormat format, bool noMilliseconds) const
-{
-    string result;
-    toString(result, format, noMilliseconds);
-    return result;
-}
-
 /*!
  * \brief Returns the string representation of the current instance using the specified \a format.
  * \remarks If \a noMilliseconds is true the date will be rounded to full seconds.
diff --git a/chrono/datetime.h b/chrono/datetime.h
index f96b338..298c6b1 100644
--- a/chrono/datetime.h
+++ b/chrono/datetime.h
@@ -61,9 +61,9 @@ public:
     static DateTime fromIsoStringGmt(const char *str);
     static DateTime fromIsoStringLocal(const char *str);
     static DateTime fromTimeStamp(time_t timeStamp);
-    static DateTime fromTimeStampGmt(time_t timeStamp);
+    constexpr static DateTime fromTimeStampGmt(time_t timeStamp);
 
-    std::uint64_t &ticks();
+    constexpr std::uint64_t &ticks();
     constexpr std::uint64_t totalTicks() const;
     int year() const;
     int month() const;
@@ -212,10 +212,18 @@ inline DateTime DateTime::fromIsoStringLocal(const char *str)
     return fromIsoString(str).first;
 }
 
+/*!
+ * \brief Constructs a new DateTime object with the GMT time from the specified UNIX \a timeStamp.
+ */
+constexpr inline DateTime DateTime::fromTimeStampGmt(std::time_t timeStamp)
+{
+    return DateTime(DateTime::unixEpochStart().totalTicks() + static_cast<std::uint64_t>(timeStamp) * TimeSpan::ticksPerSecond);
+}
+
 /*!
  * \brief Returns a mutable reference to the total ticks.
  */
-inline std::uint64_t &DateTime::ticks()
+constexpr inline std::uint64_t &DateTime::ticks()
 {
     return m_ticks;
 }
@@ -304,7 +312,7 @@ constexpr inline int DateTime::millisecond() const
 /*!
  * \brief Returns the microsecond component of the date represented by this instance.
  */
-constexpr int DateTime::microsecond() const
+constexpr inline int DateTime::microsecond() const
 {
     return m_ticks / TimeSpan::ticksPerMicrosecond % 1000ul;
 }
@@ -314,7 +322,7 @@ constexpr int DateTime::microsecond() const
  * \remarks The accuracy of the DateTime class is 100-nanoseconds. Hence the returned value
  *          will always have two zeros at the end (in decimal representation).
  */
-constexpr int DateTime::nanosecond() const
+constexpr inline int DateTime::nanosecond() const
 {
     return m_ticks % 10ul * TimeSpan::nanosecondsPerTick;
 }
@@ -376,6 +384,18 @@ constexpr inline bool DateTime::isSameDay(const DateTime &other) const
     return (m_ticks / TimeSpan::ticksPerDay) == (other.m_ticks / TimeSpan::ticksPerDay);
 }
 
+/*!
+ * \brief Returns the string representation of the current instance using the specified \a format.
+ * \remarks If \a noMilliseconds is true the date will be rounded to full seconds.
+ * \sa toIsoString() for ISO format
+ */
+inline std::string DateTime::toString(DateTimeOutputFormat format, bool noMilliseconds) const
+{
+    std::string result;
+    toString(result, format, noMilliseconds);
+    return result;
+}
+
 /*!
  * \brief Constructs a new instance of the DateTime class with the maximal number of ticks.
  */
diff --git a/chrono/timespan.cpp b/chrono/timespan.cpp
index 128c137..65d5361 100644
--- a/chrono/timespan.cpp
+++ b/chrono/timespan.cpp
@@ -70,19 +70,6 @@ TimeSpan TimeSpan::fromString(const char *str, char separator)
     }
 }
 
-/*!
- * \brief Converts the value of the current TimeSpan object to its equivalent std::string representation
- *        according the given \a format.
- *
- * If \a fullSeconds is true the time interval will be rounded to full seconds.
- */
-string TimeSpan::toString(TimeSpanOutputFormat format, bool fullSeconds) const
-{
-    string result;
-    toString(result, format, fullSeconds);
-    return result;
-}
-
 /*!
  * \brief Converts the value of the current TimeSpan object to its equivalent std::string representation
  *        according the given \a format.
diff --git a/chrono/timespan.h b/chrono/timespan.h
index 65f51b5..6726287 100644
--- a/chrono/timespan.h
+++ b/chrono/timespan.h
@@ -375,6 +375,19 @@ inline TimeSpan &TimeSpan::operator-=(const TimeSpan &other)
     return *this;
 }
 
+/*!
+ * \brief Converts the value of the current TimeSpan object to its equivalent std::string representation
+ *        according the given \a format.
+ *
+ * If \a fullSeconds is true the time interval will be rounded to full seconds.
+ */
+inline std::string TimeSpan::toString(TimeSpanOutputFormat format, bool fullSeconds) const
+{
+    std::string result;
+    toString(result, format, fullSeconds);
+    return result;
+}
+
 /*!
  * \brief Returns ture if the time interval represented by the current TimeSpan class is null.
  */