From FIXwiki

FIX Data Type UTCTimeOnly

Name	BaseType	First introduced	Deprecated
UTCTimeOnly	String	FIX.4.2

string field representing Time-only represented in UTC (Universal Time Coordinated, also known as "GMT") in either HH:MM:SS (whole seconds) or HH:MM:SS.sss (milliseconds) format, colons, and period required. This special-purpose field is paired with UTCDateOnly to form a proper UTCTimestamp for bandwidth-sensitive messages.

Valid values:

HH = 00-23, MM = 00-60 (60 only if UTC leap second), SS = 00-59. (without milliseconds)

HH = 00-23, MM = 00-59, SS = 00-60 (60 only if UTC leap second), sss=000-999 (indicating milliseconds).

Notes

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