NIST maintains the ACTS telephone time service. I happened to record a call made to that phone number during the leap second at the end of 1998. ACTS transmits one line per second; notice the line with the time of 23:59:60. The "LS" (Leap Second) field changes from 1 (leap second pending) to 0 (no leap second pending) after that line.
The UT1 correction during that day was -0.3 seconds from which one can conclude the pending leap second event will be an insertion (positive leap second) rather than a deletion (negative leap second).
One would assume that the UT1 correction changes from -0.3 to +0.7 after the leap second. But in talking with NIST, although the leap second pending bit is handled automatically, the UT1 correction is a manual change and the engineer at the WWVB station did not change the UT1 setting until some point later on.
From: Tom Van Baak <tvb veritas.com>
Subject: Leap Second, ACTS
Date: 1999/01/01
Message-ID: <368D1090.51F6@veritas.com>#1/1
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Here's an ACTS session with yesterday's leap second.
/tvb
ATDT13034944774
CONNECT 1200
National Institute of Standards and Technology
Telephone Time Service, Generator 1c
Enter the question mark character for HELP
D L
MJD YR MO DA HH MM SS ST S UT1 msADV <OTM>
51178 98-12-31 23:59:49 00 1 -.3 045.0 UTC(NIST) *
51178 98-12-31 23:59:50 00 1 -.3 045.0 UTC(NIST) *
51178 98-12-31 23:59:51 00 1 -.3 045.0 UTC(NIST) *
51178 98-12-31 23:59:52 00 1 -.3 045.0 UTC(NIST) *
51178 98-12-31 23:59:53 00 1 -.3 045.0 UTC(NIST) *
51178 98-12-31 23:59:54 00 1 -.3 045.0 UTC(NIST) *
51178 98-12-31 23:59:55 00 1 -.3 045.0 UTC(NIST) *
51178 98-12-31 23:59:56 00 1 -.3 045.0 UTC(NIST) *
51178 98-12-31 23:59:57 00 1 -.3 045.0 UTC(NIST) *
51178 98-12-31 23:59:58 00 1 -.3 045.0 UTC(NIST) *
51178 98-12-31 23:59:59 00 1 -.3 045.0 UTC(NIST) *
51178 98-12-31 23:59:60 00 1 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:00 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:01 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:02 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:03 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:04 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:05 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:06 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:07 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:08 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:09 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:10 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:11 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:12 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:13 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:14 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:15 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:16 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:17 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:18 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:19 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:20 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:21 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:22 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:23 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:24 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:25 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:26 00 0 -.3 045.0 UTC(NIST) *
51179 99-01-01 00:00:27 00 0 -.3 045.0 UTC(NIST) *
NO CARRIER
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The above was an old posting of mine found in the Google Usenet archives.