Fix strict mode errors in built-ins/Date

test/built-ins/Date/UTC/S15.9.4.3_A1_T1.js

test/built-ins/Date/UTC/S15.9.4.3_A1_T3.js

test/built-ins/Date/UTC/S15.9.4.3_A3_T1.js

test/built-ins/Date/UTC/S15.9.4.3_A3_T3.js

test/built-ins/Date/parse/S15.9.4.2_A1_T1.js

test/built-ins/Date/parse/S15.9.4.2_A1_T3.js

test/built-ins/Date/parse/S15.9.4.2_A3_T1.js

test/built-ins/Date/parse/S15.9.4.2_A3_T3.js

test/built-ins/Date/prototype/S15.9.4.1_A1_T1.js

test/built-ins/Date/prototype/S15.9.4.1_A1_T2.js

test/built-ins/Date/prototype/S15.9.4.1_A1_T3.js

test/built-ins/Date/prototype/constructor/S15.9.5.1_A1_T1.js

test/built-ins/Date/prototype/constructor/S15.9.5.1_A1_T3.js

test/built-ins/Date/prototype/constructor/S15.9.5.1_A3_T1.js

test/built-ins/Date/prototype/constructor/S15.9.5.1_A3_T3.js

test/built-ins/Date/prototype/getDate/S15.9.5.14_A1_T1.js

test/built-ins/Date/prototype/getDate/S15.9.5.14_A1_T3.js

test/built-ins/Date/prototype/getDate/S15.9.5.14_A3_T1.js

test/built-ins/Date/prototype/getDate/S15.9.5.14_A3_T3.js

test/built-ins/Date/prototype/getDay/S15.9.5.16_A1_T1.js