Unexpected date formats can result in index errors

Problem/Motivation

The logic for DateTimeOffsets can encounter an indexing error when a date format does not have `datetime_type` set but is also not a timestamp. An example is an aggregated date field.

Steps to reproduce

1. Add an aggregated date field to the index that includes a date field not stored in timestamp format. Select `Date Time offset` as the type.
2. Attempt to run an index.

Items indexed successfully with date field populated.

Error is encountered at `createFromTimestamp` as date might not be a timestamp value.

Proposed resolution

Add a generic `strtotime` conversion in case a value is not a timestamp but also does not have `datetime_type` available in field settings.

Remaining tasks

User interface changes

API changes

Data model changes