SQLite Light

hi - thanks for looking.

would you please help me with the following sql statement using sqlite?

table d
ID int    
trxDate text
(no records in table - the text field is a string of date/time)

table t
ID int    
trxDate text

values for table t:
1, '11/15/2017 13:41'
2, '11/15/2017 13:45'

i need the following:

max of the date/time from table d.  if it is null, return the max date/time from table t.  if that is null, return the current date/time.

i tried to use the IFNULL function, but the second part of the function doesn't seem to be able to have a new SELECT statement within it.

any help would be appreciated.... thank you.