diff --git a/.gitignore b/.gitignore
index aaec9ef..daeb3d3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,4 @@
-*.c
+vieter.c
 /data/
 
 # Build artifacts
diff --git a/src/cron/expression/c/expression.h b/src/cron/expression/c/expression.h
index 079a72a..5b8e4f9 100644
--- a/src/cron/expression/c/expression.h
+++ b/src/cron/expression/c/expression.h
@@ -1,5 +1,7 @@
 #include <time.h>
 #include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
 
 typedef struct cron_expression {
     uint8_t *minutes;
diff --git a/src/cron/expression/c/parse.c b/src/cron/expression/c/parse.c
new file mode 100644
index 0000000..17d416f
--- /dev/null
+++ b/src/cron/expression/c/parse.c
@@ -0,0 +1,179 @@
+#include "expression.h"
+
+const uint8_t min[4] = {0, 0, 1, 1};
+const uint8_t max[4] = {59, 23, 31, 12};
+
+typedef enum parse_error {
+    ParseOk = 0,
+    ParseInvalidExpression = 1,
+    ParseInvalidNumber = 2,
+    ParseOutOfRange = 3
+} ParseError;
+
+#define SAFE_ATOI(v,s,min,max) \
+    int _##v = atoi(s); \
+    if ((_##v) == 0 && strcmp((s), "0") != 0) { \
+        return ParseInvalidNumber; \
+    } \
+    if (v < (min) || v > (max)) { \
+        return ParseOutOfRange; \
+    } \
+    v = (uint8_t) (_##v);
+
+/**
+ * Given a range expression, produce a bit field defining what numbers in the
+ * min-max range the expression represents. The first bit (starting from the
+ * right) corresponds to min, the max - min + 1'th bit to max. All trailing bits
+ * after this should be ignored. The given bitfield is modified in-place, so
+ * multiple calls of this function can be performed on the same value to create
+ * the effect of ORing their values:
+ *
+ * A range expression has one of the following forms:
+ *
+ * - *
+ * - a
+ * - a-b
+ * - a/c
+ * - a-b/c
+ */
+ParseError ce_parse_range(uint64_t *out, char *s, uint8_t min, uint8_t max) {
+    // The * expression means "every possible value"
+    if (s[0] == '*') {
+        // A '*' is only valid on its own
+        if (s[1] != '\0') {
+            return ParseInvalidExpression;
+        }
+
+        *out = ~0;
+
+        return ParseOk;
+    }
+
+    size_t slash_index = 0;
+    size_t dash_index = 0;
+    size_t i = 0;
+
+    // We first iterate over the string to determine whether it contains a slash
+    // and/or a dash. We know the dash can only be valid if it appears before
+    // the slash.
+    while (s[i] != '\0' && slash_index == 0) {
+        if (s[i] == '/') {
+            slash_index = i;
+
+            s[i] = '\0';
+        } else if (s[i] == '-') {
+            dash_index = i;
+
+            s[i] = '\0';
+        }
+
+        i++;
+    }
+
+    // Parse the three possible numbers in the pattern
+    uint8_t start = 0;
+    uint8_t end = 0;
+    uint8_t interval = 1;
+
+    SAFE_ATOI(start, s, min, max);
+
+    if (dash_index > 0) {
+        SAFE_ATOI(end, &s[dash_index + 1], min, max);
+    }
+
+    if (slash_index > 0) {
+        SAFE_ATOI(interval, &s[slash_index + 1], 1, max - min);
+    }
+
+    // Single number doesn't need to loop
+    if (end == 0 && slash_index == 0) {
+        *out |= 1 << (start - min);
+    } else {
+        for (;start <= end; start += interval) {
+            *out |= 1 << (start - min);
+            start += interval;
+        }
+    }
+
+    return ParseOk;
+}
+
+ParseError ce_parse_part(uint64_t *out, char *s, uint8_t min, uint8_t max) {
+    *out = 0;
+
+    char *next;
+    ParseError res;
+    
+    while ((next = strchr(s, ',')) != NULL) {
+        next[0] = '\0';
+        res = ce_parse_range(out, s, min, max);
+
+        if (res != ParseOk) {
+            return res;
+        }
+
+        s = next + 1;
+    }
+
+    // Make sure to parse the final range as well
+    res = ce_parse_range(out, s, min, max);
+
+    if (res != ParseOk) {
+        return res;
+    }
+
+    return ParseOk;
+}
+
+ParseError ce_parse_expression(uint64_t *out, char *s) {
+    uint8_t part_count = 0;
+
+    char *next;
+    ParseError res;
+
+    // Skip leading spaces
+    while (s[0] == ' ') {
+        s++;
+    }
+    
+    while (part_count < 4 && (next = strchr(s, ' ')) != NULL) {
+        next[0] = '\0';
+        res = ce_parse_part(&out[part_count], s, min[part_count], max[part_count]);
+
+        if (res != ParseOk) {
+            return res;
+        }
+
+        s = next + 1;
+        size_t offset = 1;
+
+        // Skip multiple spaces
+        while (next[offset] == ' ') {
+            offset++;
+        }
+        s = next + offset;
+
+        part_count++;
+    }
+
+    // Parse final trailing part
+    if (part_count < 4 && s[0] != '\0') {
+        // Make sure to parse the final range as well
+        res = ce_parse_part(&out[part_count], s, min[part_count], max[part_count]);
+
+        if (res != ParseOk) {
+            return res;
+        }
+
+        part_count++;
+    }
+
+    // Ensure there's always 4 parts, as expressions can have between 2 and 4 parts
+    while (part_count < 4) {
+        // Expression is augmented with '*' expressions
+        out[part_count] = ~0;
+        part_count++;
+    }
+
+    return ParseOk;
+}