As aholzer said in the comments above, "$selectedStartDateFrom$"="" is wrong, so I changed that to len("$selectedStartDateFrom$")>0 . I also needed to move the if statements to before the date comparison for the startDateFrom and startDateTo to be available. Also start date of 01/01/1970 returns no results, so I changed it to a later date.
| eval startDateFrom=if(len("$selectedStartDateFrom$")>0, "$selectedStartDateFrom$", "01/01/2000")
| eval startDateTo=if(len("$selectedStartDateTo$")>0, "$selectedStartDateTo$", "01/01/2099")
| eval compare=strptime(Experiment_Instance_Start_Date,"%m/%d/%Y")
| where compare>=strptime(startDateFrom,"%m/%d/%Y")
| eval compare=strptime(startDateTo,"%m/%d/%Y")
| where compare>=strptime(Experiment_Instance_Start_Date,"%m/%d/%Y")
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