mosquitto_sub will quit with an error on Windows if %U is used.

Seconds since the Unix epoch is not a parameter that can be obtained
with strftime on Windows.

Closes #1908. Thanks to Danil intl.

ChangeLog.txt

@@ -9,6 +9,10 @@ Broker:
 Client library:
 - Fix build on Mac Big Sur. Closes #1905.
 
+Clients:
+- mosquitto_sub will now quit with an error if the %U option is used on
+  Windows, rather than just quitting. Closes #1908.
+
 
 1.6.12 - 2020-08-19
 ===================

client/client_shared.c

@@ -77,6 +77,10 @@ static int check_format(const char *str)
 					// JSON output, assuming JSON payload
 				}else if(str[i+1] == 'U'){
 					// Unix time+nanoseconds
+#ifdef WIN32
+					fprintf(stderr, "Error: The %%U format option is not supported on Windows.\n");
+					return 1;
+#endif
 				}else if(str[i+1] == 'x' || str[i+1] == 'X'){
 					// payload in hex
 				}else{