Add time module to ocean framework

This computes a time interval from days and/or seconds, including
a decimal fraction.

compass/ocean/time.py

@@ -0,0 +1,38 @@
+import time
+
+import numpy as np
+
+
+def get_time_interval_string(days=None, seconds=None):
+    """
+    Convert a time interval in days and/or seconds to a string for use in a
+    model config option.  If both are provided, they will be added
+
+    Parameters
+    ----------
+    days : float, optional
+        A time interval in days
+
+    seconds : float, optional
+        A time interval in seconds
+
+    Returns
+    -------
+    time_str : str
+        The time as a string in the format "DDDD_HH:MM:SS.SS"
+
+    """
+    sec_per_day = 86400
+    total = 0.
+    if seconds is not None:
+        total += seconds
+    if days is not None:
+        total += sec_per_day * days
+
+    day_part = int(total / sec_per_day)
+    sec_part = total - day_part * sec_per_day
+    sec_decimal = sec_part - np.floor(sec_part)
+    # https://stackoverflow.com/a/1384565/7728169
+    seconds_str = time.strftime('%H:%M:%S', time.gmtime(sec_part))
+    time_str = f'{day_part:04d}_{seconds_str}.{int(sec_decimal * 1e3):03d}'
+    return time_str