A Singaporean man, who left for Malaysia when he was about a year old, eventually returned to serve his national service in 2018.
By then, Basil Lim Boon Hoh, 28, had defaulted on his NS obligations for almost 10 years.
Lim, who also has Malaysian citizenship, pleaded guilty on Thursday to two charges under the Enlistment Act.
Two other similar charges will be taken into account during his sentencing, expected to be on Oct 15.
Court documents state that Lim had remained outside of Singapore without a valid exit permit for a total of nine years, 11 months and 16 days.
His mother is a Malaysian, while his father, with whom he is no longer in contact, is a Singaporean.
The court heard that Lim is Singaporean by birth but left for Malaysia when he was about a year old.
While he was granted Malaysian citizenship in 2001, his mother still applied for a Singapore NRIC on his behalf in 2007 because she wanted him to have a choice in the future, court documents state.
Lim's father informed him of his NS obligations before he turned 10.
But his mother told him when he was 16 that he could ignore these obligations as he had Malaysian citizenship.
Court documents state that two notices were sent in 2009 to a Malaysian address that Lim's mother had provided to the Central Manpower Base (CMPB).
One required him to register online for his NS, while the other informed him that he had to report at CMPB. Lim failed to do both.
In 2016, Lim's lawyer wrote to CPMB asking for his client to be allowed to renounce his Singapore citizenship. This was rejected.
Lim returned to Singapore in March 2018 and enlisted in August that year. He completed his NS in August this year.
The court heard that Lim now wants to keep his Singapore citizenship as he does not speak Malay well and intends to work here.
On Thursday, Deputy Public Prosecutor Sean Teh urged the court to jail Lim for five months and 18 days, arguing that Lim had reported to CMPB at his own timing.
His surrender was therefore "tactical and calculated", and not out of "genuine remorse", the DPP said.
Quoting Lim's mother in his mitigation plea, defence lawyer Lim Kia Tong said she had provided CMPB with the address of a previous home in Malaysia and did not receive any physical letters on her son's NS obligations.
She also hid this from Lim as she did not want him to return to Singapore, the lawyer added.
The Ministry of Defence previously said that 14 NS defaulters have been jailed since the High Court set out a sentencing framework for such defaulters in 2017.