time-util: introduce parse_gmtoff() helper function

src/basic/time-util.c

@@ -624,6 +624,19 @@ char* format_timespan(char *buf, size_t l, usec_t t, usec_t accuracy) {
         return buf;
 }
 
+int parse_gmtoff(const char *t, long *ret) {
+        assert(t);
+
+        struct tm tm;
+        const char *k = strptime(t, "%z", &tm);
+        if (!k || *k != '\0')
+                return -EINVAL;
+
+        if (ret)
+                *ret = tm.tm_gmtoff;
+        return 0;
+}
+
 static int parse_timestamp_impl(
                 const char *t,
                 size_t max_len,

src/basic/time-util.h

@@ -149,6 +149,7 @@ static inline char* format_timestamp(char *buf, size_t l, usec_t t) {
 #define FORMAT_TIMESTAMP_STYLE(t, style) \
         format_timestamp_style((char[FORMAT_TIMESTAMP_MAX]){}, FORMAT_TIMESTAMP_MAX, t, style)
 
+int parse_gmtoff(const char *t, long *ret);
 int parse_timestamp(const char *t, usec_t *ret);
 
 int parse_sec(const char *t, usec_t *ret);