diff --git a/src/date/date.c b/src/date/date.c
index 6e6d777..e7a3a87 100644
--- a/src/date/date.c
+++ b/src/date/date.c
@@ -1,6 +1,338 @@
+#define _POSIX_C_SOURCE 200112L // for setenv
+
+#include <ctype.h>
+#include <errno.h>
+#include <stdbool.h>
 #include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+
+#define NAME "date (canoutils)"
+#define VERSION "1.0.0"
+#define AUTHOR "Eike Flath"
+
+#include "version_info.h"
+
+#define INITIAL_BUF_SIZE 512
+
+static char *operand;
+
+static struct tm *get_time(long *nanos) {
+  struct timespec ts;
+  if (clock_gettime(CLOCK_REALTIME, &ts)) {
+    fprintf(stderr, "date: failed to retrieve time: %s\n", strerror(errno));
+    exit(EXIT_FAILURE);
+  }
+  if (nanos)
+    *nanos = ts.tv_nsec;
+  return localtime(&ts.tv_sec);
+}
+
+static int set_time(char *str) {
+  size_t n;
+  for (n = 0; str[n]; n++) {
+    if (!isdigit(str[n])) {
+      fprintf(stderr, "date: invalid date '%s'\n", str);
+      return EXIT_FAILURE;
+    }
+  }
+
+#define s(i) (str[i] - '0')
+  struct tm tm;
+  long ns;
+  memcpy(&tm, get_time(&ns), sizeof tm);
+  bool century_specified = false;
+  switch (n) {
+  case 12:
+    tm.tm_year = 100 * (10 * s(n - 4) + s(n - 3));
+    century_specified = true;
+    // fallthrough
+  case 10:
+    if (century_specified) {
+      tm.tm_year += 10 * s(n - 2) + s(n - 1);
+    } else {
+      int yy = 10 * s(n - 2) + s(n - 1);
+      int cc = yy >= 69 ? 19 : 20;
+      tm.tm_year = 100 * cc + yy;
+    }
+    tm.tm_year -= 1900;
+    // fallthrough
+  case 8:
+    tm.tm_mon = 10 * s(0) + s(1) - 1; // tm.tm_mon is in [0,11]
+    tm.tm_mday = 10 * s(2) + s(3);
+    tm.tm_hour = 10 * s(4) + s(5);
+    tm.tm_min = 10 * s(6) + s(7);
+    tm.tm_isdst = -1;
+    break;
+  default:
+    fprintf(stderr, "date: invalid date '%s'\n", str);
+    return EXIT_FAILURE;
+  }
+#undef s
+  time_t t = mktime(&tm);
+  if (t == -1) {
+    fprintf(stderr, "date: '%s' does not specify a valid time\n", str);
+    return EXIT_FAILURE;
+  }
+  struct timespec ts = {.tv_sec = t, .tv_nsec = ns};
+  if (clock_settime(CLOCK_REALTIME, &ts)) {
+    fprintf(stderr, "date: cannot set date: %s\n", strerror(errno));
+    return EXIT_FAILURE;
+  }
+  return EXIT_SUCCESS;
+}
+
+static void set_operand(char *o) {
+  if (operand) {
+    fprintf(stderr, "date: multiple output formats or dates specified\n");
+    exit(EXIT_FAILURE);
+  }
+  operand = o;
+}
+
+static void use_utc(void) {
+  if (setenv("TZ", "UTC", 1) == -1) {
+    fprintf(stderr, "date: cannot setenv TZ: %s\n", strerror(errno));
+    exit(EXIT_FAILURE);
+  }
+}
+
+static void use_iso_fmt(char *iso) {
+  if (!strcmp(iso, "hours")) {
+    set_operand("+%FT%H%:z");
+  } else if (!strcmp(iso, "minutes")) {
+    set_operand("+%FT%H:%M%:z");
+  } else if (!strcmp(iso, "date")) {
+    set_operand("+%F");
+  } else if (!strcmp(iso, "seconds")) {
+    set_operand("+%FT%T%:z");
+  } else if (!strcmp(iso, "ns")) {
+    set_operand("+%FT%T,%N%:z");
+  } else {
+    fprintf(stderr, "date: invalid argument '%s' for '--iso-8601'\n", iso);
+    fprintf(stderr,
+            "must be one of 'hours', 'minutes', 'date', 'seconds', 'ns'\n");
+    exit(EXIT_FAILURE);
+  }
+}
+
+static void use_rfc3339_fmt(char *rfc) {
+  if (!strcmp(rfc, "date")) {
+    set_operand("+%F");
+  } else if (!strcmp(rfc, "seconds")) {
+    set_operand("+%F %T%:z");
+  } else if (!strcmp(rfc, "ns")) {
+    set_operand("+%F %T.%N%:z");
+  } else {
+    fprintf(stderr, "date: invalid argument '%s' for '--rfc-3339'\n", rfc);
+    fprintf(stderr, "must be one of 'date', 'seconds', 'ns'\n");
+    exit(EXIT_FAILURE);
+  }
+}
+
+static void use_rfc_mail_fmt(void) { set_operand("+%a, %d %b %Y %T %z"); }
+
+static void short_opt(char *opt) {
+  switch (*opt) {
+  case 'u':
+    use_utc();
+    break;
+  case 'I':
+    if (opt[1])
+      use_iso_fmt(opt + 1);
+    else
+      use_iso_fmt("date");
+    break;
+  case 'R':
+    use_rfc_mail_fmt();
+    break;
+  default:
+    fprintf(stderr, "date: unknown option '-%c'\n", *opt);
+    exit(EXIT_FAILURE);
+  }
+}
+
+static bool strpre(const char *pre, const char *str) {
+  return !strncmp(pre, str, strlen(pre));
+}
+
+static void long_opt(char *opt) {
+  if (!strcmp(opt, "version")) {
+    print_version();
+    exit(EXIT_SUCCESS);
+  }
+  if (!strcmp(opt, "help"))
+    exit(system("man date"));
+  if (!strcmp(opt, "utc") || !strcmp(opt, "universal")) {
+    use_utc();
+    return;
+  }
+  if (!strcmp(opt, "rfc-email")) {
+    use_rfc_mail_fmt();
+    return;
+  }
+  if (strpre("iso-8601", opt)) {
+    char *arg = opt + strlen("iso-8601");
+    if (*arg) {
+      if (*arg != '=')
+        goto unknown_opt;
+      use_iso_fmt(arg + 1);
+    } else {
+      use_iso_fmt("date");
+    }
+    return;
+  }
+  if (strpre("rfc-3339", opt)) {
+    char *arg = opt + strlen("rfc-3339");
+    if (*arg != '=')
+      goto unknown_opt;
+    use_rfc3339_fmt(arg + 1);
+    return;
+  }
+unknown_opt:
+  fprintf(stderr, "date: unknown option '--%s'\n", opt);
+  exit(EXIT_FAILURE);
+}
+
+/**
+ * Works just like strftime, except it recognizes formats like %N or %:z which
+ * are specified by the GNU date utility but not by the glibc strftime
+ * implementation.
+ *
+ * Currently missing: %::z, %:::z
+ */
+static size_t extended_strftime(char *restrict buf, size_t max,
+                                const char *restrict fmt,
+                                const struct tm *restrict tm, long nanos) {
+  size_t fmt_size = strlen(fmt) + 1;
+  char *f = malloc(fmt_size);
+  if (!f) {
+    fprintf(stderr, "date: malloc failed: %s\n", strerror(errno));
+    exit(EXIT_FAILURE);
+  }
+  // copy the fmt string into buffer so that we can mutate it
+  memcpy(f, fmt, fmt_size);
+  size_t n = 0;
+  char c;
+  int i, j = 0;
+  for (i = 0; (c = f[i]); i++) {
+    if (c != '%')
+      continue;
+    switch (f[i + 1]) {
+    case '%':
+      i++; // skip additional %
+      continue;
+    case ':':
+      if (f[i + 2] != 'z')
+        continue;
+      // fallthrough
+    case 'N':
+      if (i > j) {
+        // evaluate the "normal" format string until i using strftime
+        f[i] = '\0'; // tells strftime to stop at i
+        size_t m = strftime(buf, max - n, f + j, tm);
+        if (m == 0) {
+          free(f);
+          return 0;
+        }
+        n += m;
+        buf += m;
+      }
+      break;
+    default:
+      continue;
+    }
+    switch (f[i + 1]) {
+    case ':':            // means %:z
+      if (max - n < 7) { // need at least 7 bytes: +hh:mm\0
+        free(f);
+        return 0;
+      }
+      strftime(buf, 6, "%z", tm); // write +hhmm\0
+      memmove(buf + 4, buf + 3, 2);
+      buf[3] = ':';
+      n += 6;
+      buf += 6;
+      j = i + 3;
+      break;
+    case 'N':
+      if (max - n < 10) {
+        free(f);
+        return 0;
+      }
+      snprintf(buf, 10, "%09ld", nanos);
+      n += 9;
+      buf += 9;
+      j = i + 2;
+      break;
+    default:
+      __builtin_unreachable();
+    }
+    i = j - 1;
+  }
+  if (i > j) {
+    // evaluate the "normal" format string until i using strftime
+    size_t m = strftime(buf, max - n, f + j, tm);
+    if (m == 0) {
+      free(f);
+      return 0;
+    }
+    n += m;
+    buf += m;
+  }
+  *buf = '\0';
+  free(f);
+  return n;
+}
+
+int main(int argc, char **argv) {
+  operand = NULL;
+  bool parse_options = true;
+  for (int i = 1; i < argc; i++) {
+    if (parse_options && argv[i][0] == '-') {
+      if (argv[i][1] == '-') {
+        char *opt = argv[i] + 2;
+        if (!*opt)
+          parse_options = false;
+        else
+          long_opt(opt);
+        continue;
+      } else {
+        if (argv[i][1]) {
+          short_opt(argv[i] + 1);
+          continue;
+        }
+      }
+    }
+    set_operand(argv[i]);
+  }
+  if (!operand)
+    operand = "+%a %b %e %H:%M:%S %Z %Y";
+
+  if (operand[0] != '+')
+    return set_time(operand);
+
+  long ns;
+  struct tm *tm = get_time(&ns);
 
-int main(void) {
-  printf("Hello, World!\n");
-  return 0;
-}
\ No newline at end of file
+  size_t cap = INITIAL_BUF_SIZE;
+  char *buf = malloc(cap);
+  if (!buf) {
+    fprintf(stderr, "date: malloc failed: %s\n", strerror(errno));
+    return EXIT_FAILURE;
+  }
+  while (!extended_strftime(buf, cap, operand + 1, tm, ns)) {
+    cap *= 2;
+    char *new_buf = realloc(buf, cap);
+    if (!new_buf) {
+      free(buf);
+      fprintf(stderr, "date: realloc failed: %s\n", strerror(errno));
+      return EXIT_FAILURE;
+    }
+    buf = new_buf;
+  }
+  printf("%s\n", buf);
+  free(buf);
+  return EXIT_SUCCESS;
+}