The TRUNC_TIMESTAMP function returns a timestamp that is the expression truncated
to the unit specified by the format-string. If format-string is
not specified, expression is truncated to the nearest
day, as if 'DD' was specified for format-string.

.-,--'DD'----------.
>>-TRUNC_TIMESTAMP--(--expression--+------------------+--)-----><
'-,--format-string-'
- expression
- An expression that returns a value of one of the following built-in data
types: a timestamp, a character-string, or a graphic-string.
- If expression is a character or graphic string,
it must not be a CLOB or DBCLOB and its value must be a valid string representation
of a timestamp. For the valid formats of string representations of timestamps,
see String representations of datetime values.
- format-string
- An expression that returns a built-in character string data
type or graphic string data type. format-string contains
a template of how the timestamp represented by expression should
be truncated. For example, if format-string is
'DD', the timestamp that is represented by expression is
truncated to the nearest day. Leading and trailing blanks are removed from
the string, and the resulting substring must be a valid template for a timestamp.
The resulting value is then folded to uppercase, so the characters in the
value may be in any case. Allowable values for format-string are
listed in Table 1.
The result of the function is a TIMESTAMP. If either
argument can be null, the result can be null; if either argument is null,
the result is the null value.