diff --git a/.travis.yml b/.travis.yml
index ab9bcfa41..1431d0145 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,8 +3,6 @@ os:
   - osx
   - linux
 julia:
-  - "0.4"
-  - "0.5"
   - "0.6"
   - nightly
 notifications:
diff --git a/REQUIRE b/REQUIRE
index 5d4b55e56..6ab2d2aab 100644
--- a/REQUIRE
+++ b/REQUIRE
@@ -1,4 +1,4 @@
-julia 0.4
-Compat 0.25
+julia 0.6
+Compat 0.27
 Mocking 0.2
 @windows LightXML 0.2
diff --git a/appveyor.yml b/appveyor.yml
index 4037d8f45..806f5cf62 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -2,12 +2,6 @@ environment:
   JULIA_TZ_VERSION: "2016j"
 
   matrix:
-  - JULIA_URL: "https://julialang-s3.julialang.org/bin/winnt/x86/0.4/julia-0.4-latest-win32.exe"
-    COVERAGE: "true"
-  - JULIA_URL: "https://julialang-s3.julialang.org/bin/winnt/x64/0.4/julia-0.4-latest-win64.exe"
-    COVERAGE: "true"
-  - JULIA_URL: "https://julialang-s3.julialang.org/bin/winnt/x86/0.5/julia-0.5-latest-win32.exe"
-  - JULIA_URL: "https://julialang-s3.julialang.org/bin/winnt/x64/0.5/julia-0.5-latest-win64.exe"
   - JULIA_URL: "https://julialang-s3.julialang.org/bin/winnt/x86/0.6/julia-0.6-latest-win32.exe"
     COVERAGE: "true"
   - JULIA_URL: "https://julialang-s3.julialang.org/bin/winnt/x64/0.6/julia-0.6-latest-win64.exe"
diff --git a/src/conversions.jl b/src/conversions.jl
index 55a37ac1c..1bde5c53a 100644
--- a/src/conversions.jl
+++ b/src/conversions.jl
@@ -49,11 +49,11 @@ function zdt2unix(zdt::ZonedDateTime)
     Dates.datetime2unix(utc(zdt))
 end
 
-function zdt2unix{T<:Integer}(::Type{T}, zdt::ZonedDateTime)
+function zdt2unix(::Type{T}, zdt::ZonedDateTime) where T<:Integer
     floor(T, Dates.datetime2unix(utc(zdt)))
 end
 
-function zdt2unix{T<:Number}(::Type{T}, zdt::ZonedDateTime)
+function zdt2unix(::Type{T}, zdt::ZonedDateTime) where T<:Number
     convert(T, Dates.datetime2unix(utc(zdt)))
 end
 
diff --git a/src/exceptions.jl b/src/exceptions.jl
index 953f31908..75e5b2868 100644
--- a/src/exceptions.jl
+++ b/src/exceptions.jl
@@ -9,7 +9,7 @@ import Compat: @compat
 The provided local datetime is ambiguous within the specified timezone. Typically occurs on
 daylight saving time transitions which "fall back" causing duplicate hour long period.
 """
-type AmbiguousTimeError <: TimeError
+mutable struct AmbiguousTimeError <: TimeError
     local_dt::DateTime
     timezone::TimeZone
 end
@@ -26,7 +26,7 @@ The provided local datetime is does not exist within the specified timezone. Typ
 occurs on daylight saving time transitions which "spring forward" causing an hour long
 period to be skipped.
 """
-type NonExistentTimeError <: TimeError
+mutable struct NonExistentTimeError <: TimeError
     local_dt::DateTime
     timezone::TimeZone
 end
diff --git a/src/local.jl b/src/local.jl
index c27464798..1d18ed43b 100644
--- a/src/local.jl
+++ b/src/local.jl
@@ -1,9 +1,9 @@
 # Determine the local system's time zone
 # Based upon Python's tzlocal https://pypi.python.org/pypi/tzlocal
-import Compat: @static, is_apple, is_unix, is_windows, readstring
+import Compat: @static, readstring, Sys
 using Mocking
 
-if is_windows()
+if Sys.iswindows()
     import TimeZones.WindowsTimeZoneIDs: WINDOWS_TRANSLATION
 end
 
@@ -13,7 +13,7 @@ end
 Returns a `TimeZone` object that is equivalent to the system's current time zone.
 """
 function localzone()
-    @static if is_apple()
+    @static if Sys.isapple()
         name = @mock readstring(`systemsetup -gettimezone`)  # Appears to only work as root
         if contains(name, "Time Zone: ")
             name = strip(replace(name, "Time Zone: ", ""))
@@ -23,7 +23,7 @@ function localzone()
             name = match(r"(?<=zoneinfo/).*$", name).match
         end
         return TimeZone(name)
-    elseif is_unix()
+    elseif Sys.isunix()
         name = ""
         validnames = timezone_names()
 
@@ -115,7 +115,7 @@ function localzone()
                 read_tzfile(f, "local")
             end
         end
-    elseif is_windows()
+    elseif Sys.iswindows()
         # Windows powershell should be available on Windows 7 and above
         win_name = strip(@mock readstring(`powershell -Command "[TimeZoneInfo]::Local.Id"`))
 
diff --git a/src/types.jl b/src/types.jl
index ef25f472a..cd2d1497c 100644
--- a/src/types.jl
+++ b/src/types.jl
@@ -28,7 +28,7 @@ const FIXED_TIME_ZONE_REGEX = r"""
 
 A `TimeZone` with a constant offset for all of time.
 """
-immutable FixedTimeZone <: TimeZone
+struct FixedTimeZone <: TimeZone
     name::Symbol
     offset::UTCOffset
 end
@@ -89,7 +89,7 @@ function FixedTimeZone(s::AbstractString)
     return FixedTimeZone(name, offset)
 end
 
-immutable Transition
+struct Transition
     utc_datetime::DateTime  # Instant where new zone applies
     zone::FixedTimeZone
 end
@@ -101,7 +101,7 @@ Base.isless(x::Transition,y::Transition) = isless(x.utc_datetime,y.utc_datetime)
 
 A `TimeZone` with an offset that changes over time.
 """
-immutable VariableTimeZone <: TimeZone
+struct VariableTimeZone <: TimeZone
     name::Symbol
     transitions::Vector{Transition}
     cutoff::Nullable{DateTime}
@@ -126,7 +126,7 @@ end
 # A `DateTime` that includes `TimeZone` information.
 # """
 
-immutable ZonedDateTime <: TimeType
+struct ZonedDateTime <: TimeType
     utc_datetime::DateTime
     timezone::TimeZone
     zone::FixedTimeZone  # The current zone for the utc_datetime.
diff --git a/src/tzdata/archive.jl b/src/tzdata/archive.jl
index 57a46da21..11b7c05ca 100644
--- a/src/tzdata/archive.jl
+++ b/src/tzdata/archive.jl
@@ -1,6 +1,6 @@
-import Compat: @static, is_windows
+import Compat: @static, Sys
 
-if is_windows()
+if Sys.iswindows()
     const exe7z = joinpath(JULIA_HOME, "7z.exe")
 end
 
@@ -12,7 +12,7 @@ specified only the files given will be extracted. The `verbose` flag can be used
 additional information to STDOUT.
 """
 function extract(archive, directory, files=AbstractString[]; verbose::Bool=false)
-    @static if is_windows()
+    @static if Sys.iswindows()
         cmd = pipeline(`$exe7z x $archive -y -so`, `$exe7z x -si -y -ttar -o$directory $files`)
     else
         cmd = `tar xvf $archive --directory=$directory $files`
@@ -31,7 +31,7 @@ end
 Determines if the given `path` is an archive.
 """
 function isarchive(path)
-    @static if is_windows()
+    @static if Sys.iswindows()
         success(`$exe7z t $path -y`)
     else
         success(`tar tf $path`)
@@ -44,7 +44,7 @@ end
 Returns the file names contained in the `archive`.
 """
 function readarchive(archive)
-    @static if is_windows()
+    @static if Sys.iswindows()
         files = AbstractString[]
         header = "-" ^ 24
         content = false
diff --git a/src/tzdata/compile.jl b/src/tzdata/compile.jl
index d8b9b86f3..ff9b85378 100644
--- a/src/tzdata/compile.jl
+++ b/src/tzdata/compile.jl
@@ -16,7 +16,7 @@ else
 end
 
 # Zone type maps to an Olson Timezone database entity
-type Zone
+mutable struct Zone
     gmtoffset::TimeOffset
     save::TimeOffset
     rules::AbstractString
@@ -39,7 +39,7 @@ function Base.isless(x::Zone,y::Zone)
 end
 
 # Rules govern how Daylight Savings transitions happen for a given time zone
-type Rule
+mutable struct Rule
     from::Nullable{Int}  # First year rule applies
     to::Nullable{Int}    # Rule applies up until, but not including this year
     month::Int           # Month in which DST transition happens
diff --git a/src/tzdata/timeoffset.jl b/src/tzdata/timeoffset.jl
index c2235efe8..91ae1347d 100644
--- a/src/tzdata/timeoffset.jl
+++ b/src/tzdata/timeoffset.jl
@@ -3,7 +3,7 @@ import Base.Dates: Period, TimePeriod, Week, Day, Hour, Minute, Second, Millisec
     value, toms, hour, minute, second
 
 # Convenience type for working with HH:MM:SS.
-immutable TimeOffset <: TimePeriod
+struct TimeOffset <: TimePeriod
     seconds::Int
 end
 const ZERO = TimeOffset(0)
@@ -50,7 +50,9 @@ end
 
 convert(::Type{Second}, t::TimeOffset) = Second(value(t))
 convert(::Type{Millisecond}, t::TimeOffset) = Millisecond(value(t) * 1000)
-promote_rule{P<:Union{Week,Day,Hour,Minute,Second}}(::Type{P}, ::Type{TimeOffset}) = Second
+
+const WDHMS = Union{Week,Day,Hour,Minute,Second}
+promote_rule(::Type{P}, ::Type{TimeOffset}) where P<:WDHMS = Second
 promote_rule(::Type{Millisecond}, ::Type{TimeOffset}) = Millisecond
 
 # https://en.wikipedia.org/wiki/ISO_8601#Times
diff --git a/src/tzfile.jl b/src/tzfile.jl
index b206e175d..96b7da94e 100644
--- a/src/tzfile.jl
+++ b/src/tzfile.jl
@@ -6,7 +6,7 @@ import Compat: read, unsafe_string
 
 const TZFILE_MAX = unix2datetime(typemax(Int32))
 
-immutable TransitionTimeInfo
+struct TransitionTimeInfo
     gmtoff::Int32     # tt_gmtoff
     isdst::Int8       # tt_isdst
     abbrindex::UInt8  # tt_abbrind
diff --git a/src/utcoffset.jl b/src/utcoffset.jl
index 7c06c153e..e91a80b66 100644
--- a/src/utcoffset.jl
+++ b/src/utcoffset.jl
@@ -9,7 +9,7 @@ import Base.Dates: AbstractTime, Second, value
 A `UTCOffset` is an amount of time subtracted from or added to UTC to get the current
 local time – whether it's standard time or daylight saving time.
 """
-immutable UTCOffset <: AbstractTime
+struct UTCOffset <: AbstractTime
     std::Second  # Standard time offset from UTC in seconds
     dst::Second  # Daylight saving time offset in seconds
 
diff --git a/test/local.jl b/test/local.jl
index 2178c48ee..300d634e1 100644
--- a/test/local.jl
+++ b/test/local.jl
@@ -1,12 +1,12 @@
 import TimeZones: TimeZone, localzone
-import Compat: is_linux
+import Compat: Sys
 
 # Ensure that the current system's local time zone is supported. If this test fails make
 # sure to report it as an issue.
 @test isa(localzone(), TimeZone)
 
 
-if is_linux()
+if Sys.islinux()
     # Bad TZ environmental variables
     withenv("TZ" => "") do
         @test_throws ErrorException localzone()
diff --git a/test/local_mocking.jl b/test/local_mocking.jl
index 69067734d..b7b7dac33 100644
--- a/test/local_mocking.jl
+++ b/test/local_mocking.jl
@@ -1,7 +1,7 @@
 import TimeZones: TimeZone, localzone
 import Mocking: @patch, apply
 import Base: AbstractCmd
-import Compat: is_apple, is_windows, is_linux, readstring
+import Compat: Sys, readstring
 
 # For mocking make sure we are actually changing the time zone
 name = string(localzone()) == "Europe/Warsaw" ? "Pacific/Apia" : "Europe/Warsaw"
@@ -11,7 +11,7 @@ tzfile_path = joinpath(TZFILE_DIR, split(name, '/')...)
 win_name = name == "Europe/Warsaw" ? "Central European Standard Time" : "Samoa Standard Time"
 timezone = TimeZone(name)
 
-if is_apple()
+if Sys.isapple()
     # Determine time zone via systemsetup.
     patch = @patch readstring(cmd::AbstractCmd) = "Time Zone:  " * name * "\n"
     apply(patch) do
@@ -27,7 +27,7 @@ if is_apple()
         @test localzone() == timezone
     end
 
-elseif is_windows()
+elseif Sys.iswindows()
     patch = @patch readstring(cmd::AbstractCmd) = "$win_name\r\n"
     apply(patch) do
         @test localzone() == timezone
@@ -35,7 +35,7 @@ elseif is_windows()
 
     # Dateline Standard Time -> Etc/GMT+12 -> UTC-12:00
 
-elseif is_linux()
+elseif Sys.islinux()
     # Test TZ environmental variable
     withenv("TZ" => ":$name") do
         @test localzone() == timezone