added a note to explain hour placeholder in icp = next (T-??)

Author: wxtim

src/user-guide/writing-workflows/scheduling.rst

@@ -537,6 +537,10 @@ A list of times, separated by semicolons, can be provided, e.g.
 brackets, and if more than one is given, the major time unit in each (hours
 or minutes) should be of the same type.
 
+.. note::
+
+   In this context ``T-00`` "-" is a placeholder for the hours column.
+
 If an offset from the specified datetime is required, this should be
 of the form ``previous(Thh:mm) +/- PxTy`` as is used
 for determining cycle periods, e.g.
@@ -2017,7 +2021,7 @@ lowest-valued point with :term:`active` or :term:`incomplete` tasks present.
 
 The runahead limit can be set with :cylc:conf:`[scheduling]runahead limit`,
 to an integer interval (for datetime or integer cycling workflows) or to
-a datetime interval (datetime cycling only). 
+a datetime interval (datetime cycling only).
 
 The integer format ``Pn`` is an interval that spans ``n+1`` consecutive cycle
 points, regardless of how the cycle point values increment from one point to
@@ -2041,8 +2045,8 @@ finishes and allows the runahead limit to move on:
         [[graph]]
             P2 = foo  # cycle points 1, 3, 5, 7, 9, ...
 
-The active cycle points in this workflow, at some point during the run, 
-might be ``29, 31, 33, 35``, for example. 
+The active cycle points in this workflow, at some point during the run,
+might be ``29, 31, 33, 35``, for example.
 
 .. code-block:: cylc
 
@@ -2074,7 +2078,7 @@ they might be ``2060, 2061, 2062, 2063, 2064``.
 .. note::
 
    To restrict activity to a single cycle point at a time (just the base point)
-   use a null runahead interval: ``P0`` or (e.g.) ``PT0H``. 
+   use a null runahead interval: ``P0`` or (e.g.) ``PT0H``.
 
 
 .. note::