Add a note directive block at the top of datetime to describe the time zone behavior

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

docs/user/ppl/functions/datetime.rst

@@ -8,6 +8,12 @@ Date and Time Functions
    :local:
    :depth: 1
 
+.. note::
+
+    All PPL date and time functions use the UTC time zone by default, unless otherwise specified. Both input and output
+    values are interpreted as UTC. For instance, an input timestamp literal like '2020-08-26 01:01:01' is assumed to be
+    in UTC, and the now() function also returns the current date and time in UTC.
+
 ADDDATE
 -------
 
@@ -1178,7 +1184,7 @@ Example::
 
 
 MINUTE_OF_DAY
-------
+-------------
 
 Description
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