diff --git a/voussoirkit/hms.py b/voussoirkit/hms.py
new file mode 100644
index 0000000..47a2f7f
--- /dev/null
+++ b/voussoirkit/hms.py
@@ -0,0 +1,36 @@
+import math
+
+def hms_to_seconds(hms):
+    '''
+    Convert hh:mm:ss string to an integer seconds.
+    '''
+    parts = hms.split(':')
+    seconds = 0
+    if len(parts) > 3:
+        raise ValueError(f'{hms} doesn\'t match the HH:MM:SS format.')
+    if len(parts) == 3:
+        seconds += int(parts[0]) * 3600
+        parts.pop(0)
+    if len(parts) == 2:
+        seconds += int(parts[0]) * 60
+        parts.pop(0)
+    if len(parts) == 1:
+        seconds += float(parts[0])
+    return seconds
+
+def seconds_to_hms(seconds):
+    '''
+    Convert integer number of seconds to an hh:mm:ss string.
+    Only the necessary fields are used.
+    '''
+    seconds = math.ceil(seconds)
+    (minutes, seconds) = divmod(seconds, 60)
+    (hours, minutes) = divmod(minutes, 60)
+    parts = []
+    if hours:
+        parts.append(hours)
+    if hours or minutes:
+        parts.append(minutes)
+    parts.append(seconds)
+    hms = ':'.join(f'{part:02d}' for part in parts)
+    return hms