Your employer is not correct about 28 days period. However, your employer should know of any absences outside the UK and they are definitely right in asking to be notified (in fact, they have probably not been been fulfilling their obligations as a sponsor if they weren't aware of you going outside the UK).
The rule of 28 days is about leave without pay. Your leave without pay cannot exceed 28 days in a calendar year. However, you can be outside the UK for extended period if this is part of your annual holiday allowance or if the absence is work-related (which means that you are continuously employed and not on leave without pay). And in this case, you can actually be away for up to two years!
Check the section "Absences from the UK" on page 56 in the Tier 2 Policy Guidance here
https://www.gov.uk/government/uploads/s ... _04_17.pdf.