There is no built-in template, however you have described what you want an can make by following the following instructions:
1) open a new document using the menu item "File > New..."
2) Choose the category "Basic" on the left, then "Blank" from the available selections:
2) In order to facilitate entering the date and time, I suggest creating a single-cell table, then entering the formula =now(). To do this, do the following:
click and hold on cell A2 in the table (until you see the cell "lift" off the table.
Drag the cell above your table on the screen.
you should see a new table like this:
Double click the text "Table 2", then update the text so is reads "TimeStamp" (it's just a name so it's not real important). select cell A1 in the "TimeStamp" table, then type
=now()
shorthand for this is
A1=now()
Now in the original table, name the columns as "Date", "Systolic", "Diastolic", and "Notes"... like this:
now make the notes column wide by select the column for Notes (column D). To do this click the "D" at the very top of the column.
Now open the table formatter by clicking the "Format" button at the top right, then selecting the "Table" segment:
now adjust the value for the "Column" width at the bottom as needed
to enter the date in a row, select the cell in the table "TimeStamp" then select the menu item "Edit > Copy" (or use the key combination <command> + c)
now select the cell where you want to put the date/time, then select the menu item "Edit > Paste Formula Results"
now enter the reading as notes