Patchset 1/3: Create new class DateUtil

Copy TaskModel.daysBetween() and TaskModel.daysLeft() to DateUtil
Revert "Patchset 1/3: Create new class DateUtil"

This reverts commit 87db588af2cddc79a1a943fd2fdf6e4069670a96.
Revert "Patchset 0/2: Create new DateUtil class to host TaskModel.daysBetween() and TaskModel.daysLeft()"

This reverts commit 235a332c4a5907e9c2401085e95c0a20e4c8c25f.