More emphasis on UTC requirement

item-spec/item-spec.md

@@ -127,7 +127,7 @@ resources below.
 
 | Field Name | Type         | Description                                                  |
 | ---------- | ------------ | ------------------------------------------------------------ |
-| datetime   | string\|null | **REQUIRED.** The searchable date and time of the assets, in UTC. It is formatted according to [RFC 3339, section 5.6](https://tools.ietf.org/html/rfc3339#section-5.6). `null` is allowed, but requires `start_datetime` and `end_datetime` from [common metadata](common-metadata.md#date-and-time-range) to be set. |
+| datetime   | string\|null | **REQUIRED.** The searchable date and time of the assets, which must be in UTC. It is formatted according to [RFC 3339, section 5.6](https://tools.ietf.org/html/rfc3339#section-5.6). `null` is allowed, but requires `start_datetime` and `end_datetime` from [common metadata](common-metadata.md#date-and-time-range) to be set. |
 
 #### datetime
 
@@ -136,7 +136,8 @@ or representative time in the case of assets that are combined together. Though
 complex thing to capture, for this purpose keep in mind the STAC spec is primarily searching for
 data, so use whatever single date and time is most useful for a user to search for. STAC content
 extensions may further specify the meaning of the main `datetime` field, and many will also add more
-datetime fields.
+datetime fields. **All times in STAC metadata should be in [Coordinated Universal 
+Time](https://en.wikipedia.org/wiki/Coordinated_Universal_Time) (UTC).**
 If there's clearly no meaningful single 'nominal' time, it is allowed to use `null` instead.
 In this case it is **required** to specify a temporal interval with the fields `start_datetime`
 and `end_datetime` from [common metadata](common-metadata.md#date-and-time-range). For example, if