Metadata: date_submitted 
Hi,

Within my meta data I have not been including "date_submitted". To date this was not a problem, nextstrain ran to completion and an auspice json was produced.

I just updated and found if date_submitted is not included an error is made in diagnostic.py on the lines
```
to_exclude |= (rare_mutations>args.rare_mutations)&(~recent_sequences)
to_exclude |= (np.abs(clock_deviation+rare_mutations)>args.clock_plus_rare)&(~recent_sequences)
```
I have changed these back to the previous implementation and things look to be working fine. 
If this field is needed I will generate a date_submitted column and fill it was some numbers.

Thanks.
diff --git a/scripts/diagnostic.py b/scripts/diagnostic.py
index 61b0e992..203b96a3 100644
--- a/scripts/diagnostic.py
+++ b/scripts/diagnostic.py
@@ -78,11 +78,15 @@ if __name__ == '__main__':
     to_exclude |= np.abs(clock_deviation)>args.clock_filter_recent
     if check_recency:
         to_exclude |= (np.abs(clock_deviation)>args.clock_filter)&(~recent_sequences)
+        to_exclude |= (rare_mutations>args.rare_mutations)&(~recent_sequences)
+        to_exclude |= (np.abs(clock_deviation+rare_mutations)>args.clock_plus_rare)&(~recent_sequences)
+    else:
+        to_exclude |= (rare_mutations>args.rare_mutations)
+        to_exclude |= (np.abs(clock_deviation+rare_mutations)>args.clock_plus_rare)
     if check_clade_dates:
         to_exclude |= dates<clade_dates
     to_exclude |= snp_clusters>args.snp_clusters
-    to_exclude |= (rare_mutations>args.rare_mutations)&(~recent_sequences)
-    to_exclude |= (np.abs(clock_deviation+rare_mutations)>args.clock_plus_rare)&(~recent_sequences)
+
 
     if "QC_mixed_sites" in metadata.columns:
         to_exclude |= metadata.QC_mixed_sites=='bad'
['scripts/diagnostic.py']