In some cases, the Floating License Server (FLS) can show 1970 as the license end date, which is due to having an older FLS version.

If the License Server software (for both local and/or floating licenses) has not been updated to V7.x, you'll need to upgrade your Local/Floating License Server to V7.x.

After upgrading the License Server software to V7.x, licenses are not reactivated with the new License Server software V7.x, so you will need to reactivate them either through instant activation or by registering a .matkey license file.

See also:
How do I install the Floating License Server (FLS)?‍ 
How do I install licenses/register key files on the Floating License Server (FLS)?‍