diff --git a/src/datetime_io_core.cpp b/src/datetime_io_core.cpp
index 2c6351c..a65222a 100644
--- a/src/datetime_io_core.cpp
+++ b/src/datetime_io_core.cpp
@@ -5,7 +5,7 @@
 
 inline const char *skipws(const char *line) noexcept {
   const char *c = line;
-  while (*c && (*c == ' ' || *c == '/' || *c == '-' || *c == 'T' || *c == ':'))
+  while (*c && (*c == ' ' || *c == '/' || *c == '-' || *c == 'T' || *c == ':' || *c == '_'))
     ++c;
   return c;
 }
diff --git a/src/datetime_read.hpp b/src/datetime_read.hpp
index 7afe6d1..cb1eb63 100644
--- a/src/datetime_read.hpp
+++ b/src/datetime_read.hpp
@@ -20,7 +20,7 @@ template <YMDFormat F> class ReadInDate {};
 /** A Date parser expecting a Date-format of type YYYY:MM:DD
  *
  * The delimeter character (here denoted ':'), can actually be any character
- * of: ' ', '/', '-', 'T' and ':', so that a date can be given as e.g.
+ * of: ' ', '/', '-', 'T', '_' and ':', so that a date can be given as e.g.
  * "2023:10:07", or
  * "2023/10/07", or
  * "2023 10 07", or
@@ -64,7 +64,7 @@ template <> class ReadInDate<YMDFormat::YYYYMMDD> {
 /** A Date parser expecting a Date-format of type DD:MM:YYYY
  *
  * The delimeter character (here denoted ':'), can actually be any character
- * of: ' ', '/', '-', 'T' and ':', so that a date can be given as e.g.
+ * of: ' ', '/', '-', 'T', '_' and ':', so that a date can be given as e.g.
  * "07:10:2023" or
  * "07/10/2023" or
  * "07 10 2023" or
@@ -109,7 +109,7 @@ template <> class ReadInDate<YMDFormat::DDMMYYYY> {
  *  Year
  *
  * The delimeter character (here denoted ':'), can actually be any character
- * of: ' ', '/', '-', 'T' and ':', so that a date can be given as e.g.
+ * of: ' ', '/', '-', 'T', '_' and ':', so that a date can be given as e.g.
  * "2023:365" or
  * "2023-365" or
  * "2023/365" or
@@ -164,7 +164,7 @@ class ReadInTime {
 /** A Time-Of-Day parser expecting a Time-format of type HHMMSS
  *
  * The delimeter character (here denoted ':'), can actually be any character
- * of: ' ', '/', '-', 'T' and ':', so that a date can be given as e.g.
+ * of: ' ', '/', '-', 'T', '_' and ':', so that a date can be given as e.g.
  * "23:59:59", or
  * "23/59/59", or
  * "23T59T59", or
@@ -216,7 +216,7 @@ class ReadInTime<S, HMSFormat::HHMMSS> {
 /** A Time-Of-Day parser expecting a Time-format of type HHMMSS.SSSSSSS...
  *
  * The delimeter character (here denoted ':'), can actually be any character
- * of: ' ', '/', '-', 'T' and ':', so that a date can be given as e.g.
+ * of: ' ', '/', '-', 'T', '_' and ':', so that a date can be given as e.g.
  * "23:59:59.012345678912", or
  * "23/59/59.012345678912", or
  * "23T59T59.012345678912", or
diff --git a/src/datetime_write.hpp b/src/datetime_write.hpp
index ca0839a..d9c2257 100644
--- a/src/datetime_write.hpp
+++ b/src/datetime_write.hpp
@@ -22,9 +22,11 @@ template <YMDFormat F> class SpitDate {};
 template <> class SpitDate<YMDFormat::YYYYMMDD> {
 public:
   static const int numChars = 10;
-  static int spit(const ymd_date &ymd, char *buffer) noexcept {
-    return std::sprintf(buffer, "%4d/%02d/%02d", ymd.yr().as_underlying_type(),
+  static int spit(const ymd_date &ymd, char *buffer, char delimeter='/') noexcept {
+    return std::sprintf(buffer, "%4d%c%02d%c%02d", ymd.yr().as_underlying_type(),
+                        delimeter,
                         ymd.mn().as_underlying_type(),
+                        delimeter,
                         ymd.dm().as_underlying_type());
   }
 };
@@ -33,9 +35,11 @@ template <> class SpitDate<YMDFormat::YYYYMMDD> {
 template <> class SpitDate<YMDFormat::DDMMYYYY> {
 public:
   static const int numChars = 10;
-  static int spit(const ymd_date &ymd, char *buffer) noexcept {
-    return std::sprintf(buffer, "%02d/%02d/%4d", ymd.dm().as_underlying_type(),
+  static int spit(const ymd_date &ymd, char *buffer, char delimeter='/') noexcept {
+    return std::sprintf(buffer, "%02d%c%02d%c%4d", ymd.dm().as_underlying_type(),
+                        delimeter,
                         ymd.mn().as_underlying_type(),
+                        delimeter,
                         ymd.yr().as_underlying_type());
   }
 };
@@ -56,8 +60,8 @@ template <> class SpitDate<YMDFormat::DDMMYYYY> {
  *             to represent the date
  * @return On success, a pointer to \p buffer
  */
-template <YMDFormat F> const char *to_char(const ymd_date &ymd, char *buffer) {
-  if (SpitDate<F>::spit(ymd, buffer) != SpitDate<F>::numChars) {
+template <YMDFormat F> const char *to_char(const ymd_date &ymd, char *buffer, char delimeter='/') {
+  if (SpitDate<F>::spit(ymd, buffer, delimeter) != SpitDate<F>::numChars) {
     throw std::runtime_error("[ERROR] Failed to format date to string\n");
   }
   return buffer;