How to find the most dominant frequency(ies) of time series that include datetime ?

How can I find the most dominant frequency(ies) of this time series that includes datetime ?

a = {
    '17-Jun-2021 12:00:00', 25
    '18-Jun-2021 20:00:00', 21
    '19-Jun-2021 06:00:00', 31
    '20-Jun-2021 14:00:00', 35
    '21-Jun-2021 17:00:00', 31
    '22-Jun-2021 16:00:00', 41
    '23-Jun-2021 16:00:00', 22
    '24-Jun-2021 13:00:00', 22
    '25-Jun-2021 15:00:00', 45
    '26-Jun-2021 11:00:00', 20
    '27-Jun-2021 19:00:00', 25
    '28-Jun-2021 18:00:00', 25
    '29-Jun-2021 15:00:00', 29
    '30-Jun-2021 10:00:00', 31
    '01-Jul-2021 15:00:00', 20
    '02-Jul-2021 10:00:00', 28
    '03-Jul-2021 14:00:00', 31
    '04-Jul-2021 13:00:00', 24
    '05-Jul-2021 09:00:00', 24
    '06-Jul-2021 14:00:00', 25
    '07-Jul-2021 15:00:00', 40}

plot(datetime(a(:,1)),cell2mat(a(:,2)))

I guess this might be possible either with fft or pspectrum, or other tools, but how ?

[Sim]

I am trying something like this, (both solutions proposed by @Star Strider and @Image Analyst) but, apparently it is not working... any idea ?

[Sim]

anyone who can kindly help please ? :-)