More typing

app/helpers.py

@@ -94,7 +94,7 @@ def get_tags(path: str) -> Dict[str, Any]:
     )
 
 
-def get_relative_date(past_date: datetime,
+def get_relative_date(past_date: Optional[datetime],
                       reference_date: Optional[datetime] = None) -> str:
     """
     Return how long before reference_date past_date is as string.
@@ -185,7 +185,8 @@ def send_mail(to_addr, from_addr, subj, body):
         s.quit()
 
 
-def ms_to_mmss(ms: int, decimals: int = 0, negative: bool = False) -> str:
+def ms_to_mmss(ms: Optional[int], decimals: int = 0,
+               negative: bool = False) -> str:
     """Convert milliseconds to mm:ss"""
 
     minutes: int