Define the start_time field as optional in the actual data format; it may not always be available.

Resources/crash_report.proto

@@ -281,7 +281,8 @@ message CrashReport {
         /** If false, the process is being run via process-level CPU emulation (such as Rosetta). */
         required bool native = 6;
 
-        /** The start time of the process (as seconds since UNIX epoch). Required for all v1.2+ crash reports. */
+        /** The start time of the process (as seconds since UNIX epoch). The field may be
+         * ommitted if the start time can not be determined. */
         optional uint64 start_time = 7;
     }
   

Resources/crash_report_v2.proto

@@ -184,8 +184,9 @@ message CrashReport_V2 {
         /* Application process path */
         optional string path = 3;
         
-        /** The start time of the process (as seconds since UNIX epoch). */
-        required uint64 start_time = 4;
+        /** The start time of the process (as seconds since UNIX epoch). The field may be
+         * ommitted if the start time can not be determined. */
+        optional uint64 start_time = 4;
 
         /* Application parent process name */
         optional string parent_name = 5;