I got the same errors here with arduino 18, 22 and 1.01. and different Time lib versions.
/usr/share/arduino/libraries/Time/DateStrings.cpp:18:18: error: variable 'monthStr1' must be const in order to be put into read-only section by means of '__attribute__((progmem))'
The fix for my debian system was:
go back to gcc-avr version 4.3.5-1 * and
go back to avr-libc version 1.6.8-2 *
Now everything compiles just fine
* i updated to gcc-avr 4.7.0-2 and avr-libc 1.8.0-2 which broke compiling TIME libs.
Having fun again