diff --git a/stac_fastapi/types/stac_fastapi/types/rfc3339.py b/stac_fastapi/types/stac_fastapi/types/rfc3339.py
index b1d40999..6c6ff116 100644
--- a/stac_fastapi/types/stac_fastapi/types/rfc3339.py
+++ b/stac_fastapi/types/stac_fastapi/types/rfc3339.py
@@ -4,6 +4,7 @@
 from typing import Optional, Tuple, Union
 
 import iso8601
+from fastapi import HTTPException
 from pystac.utils import datetime_to_str
 
 RFC33339_PATTERN = (
@@ -45,53 +46,85 @@ def rfc3339_str_to_datetime(s: str) -> datetime:
     return iso8601.parse_date(s)
 
 
-def str_to_interval(interval: Optional[str]) -> Optional[DateTimeType]:
-    """Extract a tuple of datetimes from an interval string.
+def parse_single_date(date_str: str) -> datetime:
+    """
+    Parse a single RFC3339 date string into a datetime object.
+
+    Args:
+        date_str (str): A string representing the date in RFC3339 format.
+
+    Returns:
+        datetime: A datetime object parsed from the date_str.
+
+    Raises:
+        ValueError: If the date_str is empty or contains the placeholder '..'.
+    """
+    if ".." in date_str or not date_str:
+        raise ValueError("Invalid date format.")
+    return rfc3339_str_to_datetime(date_str)
+
+
+def validate_interval_format(values: list) -> None:
+    """
+    Validate the format of the interval string to ensure it contains at most
+    one forward slash.
 
-    Interval strings are defined by
-    OGC API - Features Part 1 for the datetime query parameter value. These follow the
-    form '1985-04-12T23:20:50.52Z/1986-04-12T23:20:50.52Z', and allow either the start
-    or end (but not both) to be open-ended with '..' or ''.
+    Args:
+        values (list): A list of strings split by '/' from the interval string.
+
+    Raises:
+        ValueError: If the interval string contains more than one forward slash.
+    """
+    if len(values) > 2:
+        raise ValueError("Interval string contains more than one forward slash.")
+
+
+def str_to_interval(interval: Optional[str]) -> Optional[DateTimeType]:
+    """
+    Extract a tuple of datetime objects from an interval string defined by the OGC API.
+    The interval can either be a single datetime or a range with start and end datetime.
 
     Args:
-        interval (str or None): The interval string to convert to a tuple of
-        datetime.datetime objects, or None if no datetime is specified.
+        interval (Optional[str]): The interval string to convert to datetime objects,
+        or None if no datetime is specified.
 
     Returns:
-        Optional[DateTimeType]: A tuple of datetime.datetime objects or None if
-        input is None.
+        Optional[DateTimeType]: A tuple of datetime.datetime objects or
+        None if input is None.
 
     Raises:
-        ValueError: If the string is not a valid interval string and not None.
+        HTTPException: If the string is not valid for various reasons such as being empty,
+        having more than one slash, or if date formats are invalid.
     """
     if interval is None:
         return None
 
     if not interval:
-        raise ValueError("Empty interval string is invalid.")
+        raise HTTPException(status_code=400, detail="Empty interval string is invalid.")
 
     values = interval.split("/")
-    if len(values) == 1:
-        # Single date for == date case
-        return rfc3339_str_to_datetime(values[0])
-    elif len(values) > 2:
-        raise ValueError(
-            f"Interval string '{interval}' contains more than one forward slash."
+    validate_interval_format(values)
+
+    try:
+        start = parse_single_date(values[0]) if values[0] not in ["..", ""] else None
+        end = (
+            parse_single_date(values[1])
+            if len(values) > 1 and values[1] not in ["..", ""]
+            else None
         )
-
-    start = None
-    end = None
-    if values[0] not in ["..", ""]:
-        start = rfc3339_str_to_datetime(values[0])
-    if values[1] not in ["..", ""]:
-        end = rfc3339_str_to_datetime(values[1])
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
 
     if start is None and end is None:
-        raise ValueError("Double open-ended intervals are not allowed.")
+        raise HTTPException(
+            status_code=400, detail="Double open-ended intervals are not allowed."
+        )
     if start is not None and end is not None and start > end:
-        raise ValueError("Start datetime cannot be before end datetime.")
-    else:
-        return start, end
+        raise HTTPException(
+            status_code=400, detail="Start datetime cannot be before end datetime."
+        )
+
+    return start, end
 
 
 def now_in_utc() -> datetime: