As mentioned by Jonathan your script is not accurately formed.
Essentially a variable value is stored in the app after successful reload. So to get the previous variable value, simply grab it before it is overwritten.
LET varLastExec = varThisExec;
LET varThisExec = Today();
Also notice that Date(Now()) does not give you an accurate date value, it will be a timestamp containing date and time. This is likely to cause you logical issues when using the value in comparisons. Today() will return the current date, or use Now() it you are looking for the timestamp.