This can be caused by several situations.
Field does not exist in dataset
This is generally caused when the field is entirely null. Fields without values will not be created in the dataset and references to those fields will error. This can be corrected by including data in the field at its source or by setting a defaultValue as described here.
Field reference is not correct
The Field_Name may not be correct. If the Field_Name has been manually entered in the Lens or Dashboard JSON, ensure that it is accurate. More information about Dashboard JSON is here.
Field has been relabeled in XMD
If you have used XMD to change the label for this field in the dataset, ensure that both the Lens/Dashboard JSON and the Dataset XMD match for this field. More information about XMD is here.
In the SAQL Reference we can see that the toDate() method is used to convert from String to Date.
"Converts a string to a date. If a formatString argument isn’t provided, the function uses the format yyyy-MM-dd HH:mm:ss."