Switch the query to SQL View (View menu, when in query design.)

In the WHERE clause you will see something like this:
   (Table1.Date1 Is Null) OR
   (Table1.Date1 Between [Forms].[GetDiags].[FromDate]
       And [Forms].[GetDiags].[ToDate])

Change it to:
   (([Forms].[GetDiags].[FromDate] Is Null) OR
   ([Forms].[GetDiags].[ToDate] Is Null) OR
   (Table1.Date1 Between [Forms].[GetDiags].[FromDate]
       And [Forms].[GetDiags].[ToDate]))

If either text box is null, the query returns all records. If both boxes
have a value, the query returns only dates between the values.

A more flexible solution might be to build the filter string dynamically
from just the boxes that have a value. Example in Method 2
   Limiting a Report to a Date Range
at:
   http://allenbrowne.com/casu-08.html

For a more comprehensive example, showing how to filter a form based on any
combination of text boxes, download this example (for Access 2000 and
later):
   http://allenbrowne.com/unlinked/Search2000.zip

One more question, what do you want to do if only one date is entered?

Assuming if one date is cleared, they will both be cleared, try:

(Between [Forms].[GetDiags].[FromDate] And [Forms].[GetDiags].[ToDate]) Or
([Forms].[GetDiags].[FromDate] Is Null And [Forms].[GetDiags].[ToDate] Is
Null)

Also, you'll need to define the parameters as Date/Time data type. To do
this, open the query in design view and go to Query|Parameters... on the
menu bar. In the dialog, enter the parameters and set the data types as
follows:

[Forms].[GetDiags].[FromDate]    Date/Time
[Forms].[GetDiags].[ToDate]        Date/Time