time-util: add new call usec_shift_clock() for converting times between clocks

We use that quite often, let's implement one clean version of it.

src/basic/time-util.c

@@ -1351,3 +1351,22 @@ unsigned long usec_to_jiffies(usec_t u) {
 
         return DIV_ROUND_UP(u , USEC_PER_SEC / hz);
 }
+
+usec_t usec_shift_clock(usec_t x, clockid_t from, clockid_t to) {
+        usec_t a, b;
+
+        if (x == USEC_INFINITY)
+                return USEC_INFINITY;
+        if (map_clock_id(from) == map_clock_id(to))
+                return x;
+
+        a = now(from);
+        b = now(to);
+
+        if (x > a)
+                /* x lies in the future */
+                return usec_add(b, usec_sub_unsigned(x, a));
+        else
+                /* x lies in the past */
+                return usec_sub_unsigned(b, usec_sub_unsigned(a, x));
+}

src/basic/time-util.h

@@ -145,6 +145,8 @@ bool clock_boottime_supported(void);
 bool clock_supported(clockid_t clock);
 clockid_t clock_boottime_or_monotonic(void);
 
+usec_t usec_shift_clock(usec_t, clockid_t from, clockid_t to);
+
 #define xstrftime(buf, fmt, tm) \
         assert_message_se(strftime(buf, ELEMENTSOF(buf), fmt, tm) > 0, \
                           "xstrftime: " #buf "[] must be big enough")