What does "Illegal assignment from String to Datetime" mean?

You assigned a String value to a Datetime variable or field. Apex never converts text automatically, so the class won't compile until you convert the value, for example with Datetime.valueOf().

What is the difference between Datetime.valueOf and Datetime.valueOfGmt?

valueOf reads the text as the user's local time; valueOfGmt reads it as GMT. In my test with a US Pacific user, the same text "2026-10-06 14:30:00" became 21:30 GMT with valueOf and 14:30 GMT with valueOfGmt.

How do I convert an ISO 8601 string like 2026-10-06T14:30:00Z?

Use (Datetime) JSON.deserialize('"2026-10-06T14:30:00.000Z"', Datetime.class). Datetime.valueOf fails on this format with "Invalid date/time".

How do I fix "Illegal assignment from String to Decimal"?

Use Decimal.valueOf('1234.56'). Remove thousands separators first: '1,234.56' throws "Invalid decimal".

How do I convert a Datetime back to a String?

Use myDatetime.format('yyyy-MM-dd HH:mm') for a custom format in the user's time zone, or String.valueOf(myDatetime) for the default format.

Compile Error: Illegal assignment from String to Datetime

The Apex compile error “Illegal assignment from String to Datetime” means you tried to put a String value into a Datetime variable or field, for example Datetime closeTime = closeText;. Apex never converts text to a Datetime automatically. Fix it by converting explicitly: Datetime.valueOf('2026-10-06 14:30:00'), Datetime.valueOfGmt(...) or Datetime.newInstance(...).

The same error appears for other types, such as “Illegal assignment from String to Decimal” or “to Date”. I reproduced the error in the Developer Console of a Salesforce Developer Edition org and tested every conversion method, including the formats that fail at runtime and the time-zone shift most tutorials don’t mention.

This Tutorial Covers: