[Gregorian]Calendar accept the value 0 to 59 as SECOND field. There are leap second that can take value of 0 to 60 -- actually, UNIX C library defines second is range from 0 to 61. Also, when calcurating UTC from/to Calendar, leapseconds should be considered.

Olson public time zone data 1999e lists,

# @(#)leapseconds 7.12

# Allowance for leapseconds added to each timezone file.

# The International Earth Rotation Service periodically uses leap seconds
# to keep UTC to within 0.9 s of UT1
# (which measures the true angular orientation of the earth in space); see
# Terry J Quinn, The BIPM and the accurate measure of time,
# Proc IEEE 79, 7 (July 1991), 894-905.
# There were no leap seconds before 1972, because the official mechanism
# accounting for the discrepancy between atomic time and the earth's rotation
# did not exist until the early 1970s.

# The correction (+ or -) is made at the given time, so lines
# will typically look like:
# Leap YEAR MON DAY 23:59:60 + R/S
# or
# Leap YEAR MON DAY 23:59:59 - R/S

# If the leapsecond is Rolling (R) the given time is local time
# If the leapsecond is Stationary (S) the given time is UTC

# Leap YEAR MONTH DAY HH:MM:SS CORR R/S
Leap 1972 Jun 30 23:59:60 + S
Leap 1972 Dec 31 23:59:60 + S
Leap 1973 Dec 31 23:59:60 + S
Leap 1974 Dec 31 23:59:60 + S
Leap 1975 Dec 31 23:59:60 + S
Leap 1976 Dec 31 23:59:60 + S
Leap 1977 Dec 31 23:59:60 + S
Leap 1978 Dec 31 23:59:60 + S
Leap 1979 Dec 31 23:59:60 + S
Leap 1981 Jun 30 23:59:60 + S
Leap 1982 Jun 30 23:59:60 + S
Leap 1983 Jun 30 23:59:60 + S
Leap 1985 Jun 30 23:59:60 + S
Leap 1987 Dec 31 23:59:60 + S
Leap 1989 Dec 31 23:59:60 + S
Leap 1990 Dec 31 23:59:60 + S
Leap 1992 Jun 30 23:59:60 + S
Leap 1993 Jun 30 23:59:60 + S
Leap 1994 Jun 30 23:59:60 + S
Leap 1995 Dec 31 23:59:60 + S
Leap 1997 Jun 30 23:59:60 + S
Leap 1998 Dec 31 23:59:60 + S