How can a JSON_VALUE be converted to a DateTime with EF Core 2.2?
The problem is that not all Convert method are supported.
In fact none of them are standardly supported - EF Core allows database providers to add CLR method and member translators for whatever they like. For instance SqlServer provider currently supports ToByte, ToDecimal, ToDouble, ToInt16, ToInt32, ToInt64 and ToString.
This means there is no database agnostic way of performing server side conversions.
Since you seem to be using SqlServer, as workaround I could suggest utilizing the implicit data conversions (currently supported by SqlServer provider) by using the "double cast" technique from my answer to a similar post, e.g.
.Where(t1 => (DateTime)(object)JsonExtensions.JsonValue(t1.SampleJson, "$.samplePath.sampleDate") >= date);(object) cast is used to avoid the C# compiler error. During the query translation, both casts will be removed and the SQL Server implicit data conversion will eventually do the job.