Looking on the internet deeply has found these results:

in early March is the most popular phrase on the web.

at early March 111,500 results on the web	in early March 1753,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	Nguyen Van Dai is one of two Christian lawyers arrested in early March, facing several accusations relating to his defence of religious freedom, including disseminating 'alleged infractions of religious liberty'. The common website that the Commission is running went live back in early March. Those measures expired in early March 2003. I visited Zagreb in early March and found that Croatia's European vocation was strong enough to settle this problem if it was hampering the accession negotiations. To contain exchange rate volatility and excessive market pressures, in early March 2007 Národná banka Slovenska intervened to weaken the currency. A seventh member of the group was arrested in early March, after being summoned to the offices of the Intelligence Ministry. A specific audit for the above mentioned project is foreseen, and is expected to commence in early March 2004. The Commissioner for International Cooperation, Humanitarian Aid and Crisis Response visited Haiti in early March.

at early March

111,500 results on the web

in early March

1753,000 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

Nguyen Van Dai is one of two Christian lawyers arrested in early March, facing several accusations relating to his defence of religious freedom, including disseminating 'alleged infractions of religious liberty'.
The common website that the Commission is running went live back in early March.
Those measures expired in early March 2003.
I visited Zagreb in early March and found that Croatia's European vocation was strong enough to settle this problem if it was hampering the accession negotiations.
To contain exchange rate volatility and excessive market pressures, in early March 2007 Národná banka Slovenska intervened to weaken the currency.
A seventh member of the group was arrested in early March, after being summoned to the offices of the Intelligence Ministry.
A specific audit for the above mentioned project is foreseen, and is expected to commence in early March 2004.
The Commissioner for International Cooperation, Humanitarian Aid and Crisis Response visited Haiti in early March.